Alcohol causes dehydration because it inhibits a hormone called anti-diuretic hormone (ADH.) This hormone has a constant level in the body and keeps you from urinating out all of the water in your body. When you drink alcohol, the level of this hormone becomes low and you urinate more. While this happens, you are also losing salt. One alcoholic drink can result in up to a pint of urine production. This effect becomes less as you drink more. But, if you drink 15 drinks in a night, you can end up well over two liters dehydrated.
Dehydration causes nausea, headache, and dizziness just by itself. Inflammation makes this even worse. It is like a double whammy. As a person gets older, the effects are more significant, as it takes longer for the body to rehydrate and get rid of the inflammation.