First off, are you sort of new to sex? Are you self lubricating or is the sex kind of dry? Do you notice any small cuts at the vaginal opening? If he is rushing things and not using lube, you could be just getting small traumas to the vagina. If this is the case, you need to tell him to slow down and do more foreplay to allow you to get more aroused or use lubrication. If this is not a factor then you may want to bring it up to your physician as many other things could come into play that no one here is equipped to answer.
Well, it's not too common to just bleed after sex even if it has been a while. But that doesn't mean it can't happen. The whole area in there is very vascular. Our cervix for example has lots of blood vessels and they can get aggravated and bleed. Keep your eye on it. If it was a one time occurrence, then write it off as a fluke. But if it keeps happening, come back and tell us and we'll help more!