help it's painful and idk why!!!

Okay so I'm 18 and I've only had sex with one guy and he's my boyfriend. Well we have a lot of sex and usually it feels really amazing. And then other times not so much and idk why. Like tonight it almost felt like the inside walls of my vagina was like torn or scratched. It was so painful we had to stop and it even hurt to sit up or sit down. Idk what happened or why it was so sore and painful bc he weren't even having sex for more than 10 minutes.