I bleed every time we have sex

It's been so embarrassing to have sex with my boyfriend lately because I bleed every single time. We've been active for four months, there's no pain and it's only been the last few weeks that I've been bleeding... A lot. Enough to freak him the hell out.  It's for sure NOT my period, I have a very regular cycle. I don't know what's happening - is this common or a serious problem?!? Sos