That's happened to me before. Orgasm makes the uterus contract anyway. They think that even serves a fertility purpose by helping it draw semen in. But cramps afterwards are fairly common, at least according to the ob-gyn book I was reading in the med school library. The strange thing is when it happens during my period, it helps ease those cramps. But when it's a real strong one at other times of the month, that sometimes causes cramps. It's worth getting it checked out, but it's probably just normal uterine behavior.