Why do I feel faint when I am hungover?

Okay so usually when I get a really bad hangover I start to feel faint and really nauseous, and one time I actually did faint, but just for a short time about 10 seconds I was unconscious. The first time this happened, I had been throwing up practically everything I ingested, even water. So I'm sure the fact that I had hardly any food or water in me led to me fainting. But the second time I felt faint, I had not thrown up at all. Can being dehydrated make you faint?