As Steve said, your brain has learned to compensate for the alcohol. It's an anti-depressant during consumption, but a depressant after the fact; it royally screws up our brain chemistry. Don't dispair, you're only a few days out! I found my mood improved fairly quickly. One thing that may (will) help is healthy eating. Even if you don't feel like eating, down some B-vitamins, cranberry juice, healthy stuff. It'll speed up your physical recovery, and as a result your psychological recovery.
Welcome to SR!