lower intestinal pain after vaginal sex

I'm emabrassed and confused. My husband and I have enjoyed "regular," vaginal sex now for years with no problem. Recently, after rather good-feeling sex, I have intestinal pain, which slowly grows in intensity. It's like his penis bruised a gut. Sitting on something hard helps, but my lower belly hurts, too. What the heck? I guess I should see a doctor for some kind of rectal scan? It never hurts without the sex, and then, only rarely. Maybe it's a positional thing? I just don't have a clue. Have you heard about this sort of thing?