Bleeding during sex?!

My boyfriend and I were having sex today (doggy style if that’s relevant?). After a couple minutes he stops and tells me he thinks I’m bleeding. I’m not supposed to start my period for another week and this has happened to me before, a couple of times actually. He was pretty deep so I’m not sure if that’s why but it’s got me pretty scared!! It wasn’t a small amount of blood either, it was a fair bit. Not sure if I started my period early or if this is something I should go see a doctor about? Pls help!!