Worst cramping after doggy-style sex!?

Today the guy I have been seeing and I had sex. We have been dating for about a month and have had sex at least 5 times before. He started to go doggy style and when we were finished I suffered from the worst abdominal cramping I've had in a long time(he does have a larger than average penis). I do suffer from endometriosis and ovarian cysts but this was awful! I was doubled over in pain. What the heck happened? Has anyone else ever had this happen?? HELP