pain after sex

Sometimes after engaging in intercourse I get ridiculously bad cramps, I'm talking can't move laying in the bath tub crying bad. I have noticed that it only happens in certain positions some of which aren't ideal for the deepest penitration which is why I am confused by this. Does anyone else experience this? I have a doctors appointment booked but I would just like some ideas of what it could be and what can help ease the pain. Thanks :)