What Causes Bleeding Post Intercourse?

I had sex Saturday night. Nothing unusual. I felt the pain of his penis hitting my cervix. As always, we changed positions when this happened. Even so, there was some bleeding afterwards and then pain. Later that night, severe pain set in. I have been in tremendous pain ever since. I haven't been able to stand up straight or lay down without curling into a ball. It hurts to sit in a chair. I am miserable and don't know why. What could have happened?