It's called the alcoholics cure. Basically what you've done is mildly overdoesd on a depressant drug (alcohol). When you come off a depressant, you come back up to normal levels of psycological/physiological function. But this coming back up makes you feel like shit. So, taking a drink when you're hungover puts that process on hold, inturrupting the painful climb back to normal. Kinda like connecting flights, where the extra drink is the airport.
It's a rumor, but I've heard from a couple people that tomatoes help for some reason. Drink some V8 or something, see if that helps. Lots of water and gatorade are totally usefull though, that always helps.
End of my drunken knowledge post.