Happened to me too, because of an unfortunate combination of me being smaller than average and my boyfriend being slightly larger. The pain was really intense for about half an hour or so... felt like someone was stabbing me in the uterus with a barbed knife. I slept on it, then when I was confident enough to move without toppling I drew myself a warm bath. That did wonders.

How long has it been since you had sex? Is the pain still as bad? A warm relaxing bath is the only solid advice I can offer, sorry - at least, that's what worked for me.