One major issue: dehydration. Excess alcohol upsets the production of vasopressin, a hormone that controls fluid balance. This sends you rushing to the ladies' room (repeatedly) the night of, and can leave your head aching the next day.
Booze also fiddles with your immune system's cytokines, chemical messengers that can trigger acute inflammation–a.k.a. pain–in your noggin and elsewhere. (A small dose of an anti-inflammatory such as ibuprofen may help.)
If you were knocking back bourbon instead of, say, vodka, your head might be pounding a little harder. In general, darker hard liquors (and most beers and dark wines) contain more congeners, chemicals that may exacerbate hangovers. Smoking cigs also multiplies the misery.
It can detox only one drink per hour, and it took a real beating last night. Don't traumatize it further with more liver stressors like acetaminophen painkillers.
In some cases, that nausea may have a surprising additional cause: It's a sign of withdrawal. Like most drugs, alcohol, when taken too often and in copious amounts, triggers give-me-more symptoms once it's out of your bloodstream. (Other such symptoms: shakiness, sweating, and anxiety.)
Your exhaustion isn't just from too few z's. Alcohol disturbs the brain's normal sleep cycles, so whatever shut-eye you got probably wasn't restorative.
Given your lagging mind and mood–not to mention pain and nausea–it's a good idea to put off important decision making until you feel better.
...which should be in anywhere from a few to a full 24 hours. Sadly, no hyped hangover prevention or cure–not bacon, not raw eggs, not tomato juice–has ever been scientifically proven to work. Your only real recourse is to wait it out–dim the lights and hydrate (plain H20 is fine; electrolyte drinks aren't any better).
Unless, of course, you are extra lucky: About 20 percent of people may be altogether resistant to hangovers.