First of all I would like to thank everyone who posted advice or encouragement. I got much more than I expected from this thread. I am truly glad I found this site and posted here. Thank you all.
Because everyone was so helpful, and seemed interested, I decided to post an update.
I talked to Jen after my classes today. On the phone she acted like nothing had happened the night before. When I brought it up and tried to apologize she said it was fine, forget it, and quickly changed the subject. After a bit of prodding, she said it was fine if I came over.
When I arrived it was quite awkward. She clearly didn't want to talk about it. She has trouble opening up as it is, and with something like this, combined with my original reaction, it must have been really hard for her. I really fucked up, and I told her so. I apologized and explained why I reacted like I did and what I was thinking, trying not to sound like I was making excuses. She just sat and listened, looking uncomfortable.
I told her I wanted her to be open with me with her feelings and desires. I assured her I wouldn't judge or think she was a freak. I said I loved her, wanted her to be happy, and was willing to give anything a shot.
Beforehand, when I was thinking through what I would say, I naively expected her to forgive and forget immediately. In my mind, we would embrace, and she would open up to me with her deepest secrets and desires; the same ones I had heartlessly rebuffed only the night before. But after I finished we both just sat there, me looking at her and she at the floor. She didn't say anything. Well, I gave it my best shot. I couldn't push her. I hugged her and said whenever you feel ready.
Shortly after, we went out to eat. At first it was pretty uncomfortable, the drive over there was at least. When we sat to eat the mood lightened quite a bit, thankfully. By the end of the meal we were pretty much back to our normal selves.
When I dropped her off I was going to leave, give her some space and time to think or whatever, but she wanted me to come up, saying her roommate wasn't going to be home until late. This was a good sign, I thought. So we sat together on her couch watching TV. She was on the right end and I was in the middle. Normally we would be very affectionate during a time like this. Once again it was a bit uncomfortable.
I felt like a high school freshman on my first date. What should I do, I thought. Should I push the tickling subject again? Maybe she wanted me to push it out of her. Maybe she wanted me to just leave her alone about it.
On a whim, I reached out and grabbed her side through her shirt and she looked over at me. I looked back at her, smiled, and wiggled my fingers a bit. Her face cracked a smile and she squirmed a bit. She was looking at me almost expectantly. I squeezed my fingers a bit harder and moved them up and down her side. She let out a laugh and reached for my hand. Once she grabbed my hand with her right, she reached out with her left for my stomach. Not this time, I was going to get her for a change. I grabbed her wrist, using my superior strength, I pushed her hand back into her body. Then I quickly grabbed her in a bear hug, pinning her arms to her sides. I picked her up, pushed onto the couch on her stomach, and sat on her butt. She grunted, and struggled a bit, but didn't say anything. I sat there for a second, and then asked if she liked being tickled. She didn't answer.
I reached my hands under her shirt, and she winced. I asked if my hands were cold and started running them up and down her sides and back. She squirmed and made quiet mmm sounds. When she stopped reacting to my cold hands, I started tickling. I went up and down her bare sides and ribs, trying different techniques. She squirmed and laughed a bit, but wasn't very ticklish. She didn't even try to grab my hands. If I ran my fingers very lightly over her sides, or on the outside of her back toward her sides, her body would shake and twitch. I got better results by reaching under her and wiggling my fingers into her abs and under her belt on the lower tummy. She started really laughing and bucking and reached back at my arms when I did this. Her waist and hips seemed ticklish but her belt and jeans were kind of in the way.
It was great fun feeling her squirm under me and getting different reactions from different areas and techniques. Her skin was soft and warm and felt good to touch as well. This was the first time I tickled Jen when she was pinned. Overall her upper half is only mildly ticklish, but she seemed to enjoy it. I did as well. I wish I could have seen her face.
I asked where else she was ticklish and she hesitated for a second. Then blurted out "my feet" and started giggling. I never tickled her feet before. I knew feet were a common ticklish area, but mine are thick skinned and not very sensitive, so I didn't know what to expect. I turned around and knelt over her calves. I tickled her feet with her socks still on first. I could feel her legs tense up as soon as I touched her foot. Digging my fingers in through her socks was pretty effective. When I took her socks off and started running my fingers back and forth she made some funny noises with her throat and scrunched her toes up. She was trying to hold it in, but I kept going and she started kicking and laughing. I tried all kinds of methods and spots on her feet, trying to find the best one. Digging in and wiggling my thumbnail around the center of her foot tickled her good. So did just randomly and quickly scratching all over. The spongy area where her toes meet her foot was ticklish too.
I thought about nibbling or licking her foot but decided against it. They were in her socks all day and were a bit sweaty and had sock fiber between her toes. Maybe I would have tried if they were cleaner.
I tickled her for probably a half hour total, mostly on her feet, with some breathers. Although at no time was she totally hysterical and breathless like I get easily. I see what you were talking about with the 'response control'" aspect of tickling. It was great fun seeing how different methods worked on different areas. I also enjoyed causing her to squirm and struggle under me.
After the tickling, we curled up on the couch together and talked for awhile. Jen felt comfortable enough to open up to me a bit. She said she liked being tickled and it turned her on. She didn't know if she prefers being tickled or tickling me. As some of you advised, I asked Jen to agree to use a safeword when tickling me which would make her stop immediately. She was fine with that. Our safeword is "uncle". She challenged me to make her use it someday. A challenge I hope to fulfill.
After talking for awhile I hugged Jen tightly and brought my hands into her stomach. She was laying on top of me with her head on my chest. I snaked my hands down under her belt and into her pants and started wiggling and gently pushing them into her lower stomach and her pubic region. She started laughing and squirming around on top of me. She couldn't pry my hands off so she shot her hands down and started squeezing my sides. I resisted the urge to do my usual routine and curl up into a defensive mode, and continued tickling her. We both laughed and wiggled around together for a bit. Eventually I had to let go and reach to protect my sides and ribs. Jen stopped, took my hands, and guided them back down. I went into her pants and felt how wet she was. This stuff really turned her on. I quickly moved us to the bedroom. I'm sure you can infer what went on in there.
We both had a wonderful time last night. I have to admit I enjoyed it more than I thought I would. It wasn't only the fact that Jen liked it that made me enjoy it. I had fun myself. Although I wasn't on the receiving end, so we'll see how that turns out. I am so pleased Jen opened up and expressed her feelings. Although she's known she's liked tickling for a long time, she is still inexperienced. I am looking forward to exploring this fetish with her. Not only will it improve our sex life, I think it will bring us closer together entirely.
Once again, thank you all for your help. This place gave me the confidence and piece of mind I needed to make this work for us. Jen thanks you as too. I hope you enjoyed this post. She has given me permission to make this post and future ones as well. So if you'd like, I can continue to make updates.