Why was I bleeding?

So my boyfriend and I snuck over to his old house (he’s in the process of moving) to take advantage that no one was home and to do stuff. He started fingering me and we started 69ing and shit right (we’re both 17 and know we shouldn’t have sex yet because we don’t want to risk a kid at this point in our lives). I was on my elbows and knees when he started fingering me pretty hard from behind and then he stopped because I had started bleeding. I thought I had finished my period a day ago so I started crying because I thought it was my fault, but he said it was his fault because his nails were longer than usual and he was going pretty rough. It wasn’t a lot of blood, but it was enough to cover his fingers. After then we went to the restroom and he brought me wipes and a pad to try to help me clean up. He feels super bad because he thinks it was his fault (he wasn’t grossed out or anything) but I think it was mine because it had just been one day that I THoUGHt I was off my period. Was he going too hard? Is it because I’m a virgin? We’re his nails too long? Was I still on my period? I wasn’t in pain, I probably wouldn’t have noticed.