Why did my boyfriend vomit blood?

[EmilyQ]

My boyfriend is 21 years old, he had quite a lot of alcohol yesterday and was quite drunk. When he got up this morning he was vomiting and said there was blood in it, he also said he was seeing stars when he was being sick. I am not sure how much blood was in it because i didn't see it and not sure what colour the blood was. is this anything to worry about? Is it the alcohol that has caused this?