Hormones, cysts. Though we need to know more about your pain (onset, duration, severity, type, location, etc), there are a few potential reasons for cramps. Most innocuous is that your muscles remain contracted following intercourse. Also, hormonal changes (and according to some even prostaglandins in semens, if unprotected sex) can lead to cramping. Possibility that it could be uterine/ovarian cyst. Take care!