Really bad pain

I am right now experiencing this pain in my lower abdomen after intercourse While having sex on my period, my significant other went really "deep" inside of me and there was a sharp pain for a second. About 2-5 minutes after sex I started getting horrible cramps... It is now coming up to 6 hours. It also hurts if I sit directly on my butt (I did not have anal sex. Please help! Is there something to help relieve the pain? How long does this last? Is it dangerous.?