Happens to me all the time if I'm suffering a really bad hangover (I wretch a lot when I brush my teeth, or if I spit a lot during swimming or running, so my chucking up reflex doesn't take much to get going!).
It's usually after the fourth or fifth hewie. I think it's probably a burst blood vessel or something from pretty violent chucking up episodes. It's only usually a little bit of blood at the end of all the spewing, usually with some of the bile that comes up. Usually get a sore gut for a day or two, and it usually goes.
If it passes in a day or two, you should be fine. If not, get to the docs. But to be fair, if you go to the docs and say "I've just been on a serious drinking binge and suddenly found I have blood when spewing up from a hangover", they probably won't stick a camera down there in panic, they'll probably give you some pills and shake their head at you. Vomiting actual blood is bad, having some blood in vomit at the end of a big spewing session isn't too much to worry bout.