It's from the movie Unfaithful. It looks like tickling but it plays in the movie as her being unable to handle the intimacy. From the way she has to put her hand over his, it looks like she's trying to control her ticklishness though.
I doubt it's a stunt belly - They have to pay someone to do that, and it would be pretty obnoxious for an actress to say to the producer "I need you to hire a double because I'm ticklish."
But since her face is not in the same scene as her stomach, it's possible she kept laughing inappropriately so they separated the sex and her reaction shots into two scenes.