I would talk to a doctor about this. You could have endometriosis, not a bad thing, but it can be if it is left untreated.
it could be having sex in the same position ALL THE TIME. sounds like you're on your back with your legs up in the air the entire time. stay away from missionary for a while. do doggystyle or from the side.
That's really strange... I only experienced pain after sex the first few times we did it. I can't offer any answers or suggestions since I'm not a doctor, but you may want to make an appointment with your doctor and have it checked out.
Good luck.