There are multiple things that can cause the pain, one of the more common ones is when your cervix gets bumped by his penis. It can get bruised and be very sore after a while. Basically every time you move and your cervix touch a vaginal wall, you will feel pain. It will heal and just go away by itself.
Take an NSAID pain killer (non steroidal anti-inflamatory) and see if it will help (Most pain killers are NSAID's today, look on the box/bottle for the letters. These are the same type you take for period pain and cramps. If it does not get better, you will have to see your doctor.