Crying during sex

My boyfriend and I usually have sex no matter what. Even when I’m on my period. He doesn’t mind it, we just shower after. So today is one of my last days having my period and I was very in the mood. But this time it hurt like really badly like really really bad to the point where I screamed at him to stop and started crying. I could tell he felt bad and so did I, i didn’t mean to make him feel like that I love him. He left my house frustrated and upset. I don’t understand why it hurt so much please help.