Bleeding during sex

I have a gf who winds up covered in blood by the time she and her fiancé have sex. She doesn't know why. She says it never hurts, but every time they have sex they both wind up covered in her blood. Now, her fiancé has some big feet, if ya know what I'm saying, and she says he fills her very well. I think he is actually hitting her cervix when they're having sex and it's opening and just letting her period blood out early. However she still has regular periods. Any advice or experience on this?