It happens to me everytime I have sex & he cums in me. Some people say it's Braxton Hicks. I think it's just the baby moving away from where he shot at lol. I think the baby's just startled & scared. Lol but who knows.
sex brings on orgasm spasms and can cause contractions in the uteris which is why your belly feels rock hard. but long as your not in severe pain or bleeding sex is good exercise and the extra blood flow to the baby is great also. and having sex up the further pregnant u are will help your cervix be ready for labor and soften too.
i have the same problem with all my pregnancys and my obgyn told me its normal and if it does become painful its because of how your positioned like when I'm on top I feel pain. and my dater said just try different positions that don't hurt and for my partner to be gentle. good luck