embarrassed and need answers!!!

*******Explicit****** Okay so today my SO were sitting on the bed cuddling this morning and he started moving his hand down my pants. well after getting far enough in one thing led to another and we were having sex. I couldn't help but notice it hurt really bad and was sore (mind you this isn't my first time) he wanted to get a little freaky and he picks me up and turns me around with my butt up in the air and starts going really fast. my period was last week and he looked down at his penis and goes "baby girl your bleeding are you Okay" I told him I was a little sore and so he stopped. he told me he was going to get me new clothes but he ended up starting a warm bath and put my clothes and a towel in the dryer for when I got out. I'm just wondering why I bleed like that any ideas?