It is possible that during sex, you tore or bruised something inside. Or, it is also possible that you have something going on in there like a cyst or something that would be irritated by sex. Some women also experience painful sex that is related to a problem with their hormones. Many women on birth control have painful sex as a side effect of the BC. The only way to be certain is to see a gynecologist. It sounds like it could be severe though, so I definately recommend seeing your doctor!