Cramps, missed period, and a virgin

Lately my body has been different it's growing. Mostly my boobs and butt have gotten bigger though. I also apparently missed my period I have had my period for 5 years and it's never happened before but I'm a virgin. The only sexual thing I've done was by myself (you get what I mean). Now I'm getting random cramps that are sometimes painful especially after I masturbate. Please someone help me tell me what could be wrong. I'm only 15.