pulling in my abdomen?

During sit ups, I've begun to feel this pull in my abdomen. When I come up for the sit up, I exhale, but after 40+, I feel as if one of my abs is about to pop out. I'm going to ask my doctor about this but I wanted to ask if anyone else experiences this. I'm rather fit for a 22 year old male and am not doing anything exotic or quick during the movement. However, I don't want this to impede any further gains. I've had a 72 hour break since my last work put (matriarch of the family died) so unless I'm mistaken, there isn't any reason to believe I'm overdoing it. Thoughts?