Why does your head spin when you rare drunk

The alcohol in the blood affects the ability of the red blood cells to carry oxygen, and your brain gets less oxygen. Its not enough to be harmful, but enough to make you feel light-headed. The same thing happens to people when they smoke their first cigarette after a long period without smoking.

I might add that  dizziness is one of the symptoms of dehydration . When you are drunk you are actually dehydrated.

True, the feeling of having a hangover the day after a drinking binge is that you are actually dehydrated. If you drink a large amount before going to sleep you can probably prevent most of the hangover.