Bleeding during sex??

Hey all- I have a question and nowhere to turn to for an answer!

The other day, I was hooking up with a guy and all of a sudden, he says "you're on your period."

Now, I haven't actually gotten my period in almost a year due to low body weight, and I'm not on birth control, so I knew that wasn't the case. I freaked out, ran to the bathroom, and there was plenty of blood. He told me he must have "started my period", which I knew couldn't have been true.

The next day, the bleeding had stopped, but something inside my body still hurt, and still does when I try to go to the bathroom.

Intense pain during sex isn't uncommon for me. Might there be an underlying problem? Thanks y'all 💓