I get cramps after I have really rough sex. Why would that be?

Uterine contractions. Sometimes during rough sex you stimulate the uterus and that leads to uterine contraction. Also if you have unprotected sex, sperm contains prostaglandines, substances that makes your uterus cramp. So this are the main reasons this is happening, I would still recommend to get an std check if you dont have one done yet.