a couple of things could have happened.
one is that during the act his penis may have bumped your cervix. that can cause you to cramp. i had a b/f one time who had a bigger than normal penis and i used to have to be sure he didn't get over zealous about sex...otherwise, i would cramp like crazy.
also, at different times of your cycle, you are more likely to have sex-induced cramping. i don't know how to explain that, but that's the way it works for me. hehe.
hope you feel better and that this type thing doesn't persist. my guess is that it's probably #1.
jasmine