Your electrolytes can get all out of whack. As you are flushing out (literally) all that excess water weight, you can lose a lot of potassium, magnesium, and sodium along with it which can often make you feel like a bad hangover.
You can use a commercial electrolyte supplement to ease this transition or you can salt your food liberally (sodium), eat a banana (potassium), and have some low sugar chocolate (magnesium). I might also suggest drinking coconut water. It is Mother Nature’s Gatorade.