Why do I bleed???

I have been having sex regularly with a guy and he's got a pretty big penis and almost every time we have sex I bleed. It's not period bleeding and it stops after an hour or so but I hate that it happens. He's not hurting me or anything and he definitely gets me wet way before he puts it in. None of the other guys I've had sex with make me bleed, we're not in a committed relationship and I've been with a couple of guys since we started sleeping together. Is it that his penis is just too big for my body or will I get used to it? It's so embarrassing that it happens and even though he doesn't make a big deal about it I'm worried he'll get tired of dealing with it.