How old where you at the time? It seems a little naive of her to think it strange.
Hell, if a hot girl grabbed my ol' boy between her elbows I'd get hard. I wouldn't need an elbow-fetish for it to make me stand to attention! That seems pretty obvious to me.
Maybe you hadn't told her you had a 'foot thing'? Could it be the case that she'd guessed and was trying to tease? In my experience girls are damn good at that sort of stuff. If that's the case I hope you made the most of it - if not you should be kicking your own ass to oblivion right now! After having 2 girlfriends (that I know of) that, at least appeared to like tickling (one of which is now my ex-wife for fuck's sake), my ass has been turned to mush by my own doing for not attempting to capitalise. A mistake I hope to never make again.