I think your body just needs to get used to the change. At first, you're having withdrawals, feel not yourself, and run down...but then I think you'll get into the swing of things.
Are you getting enough hours of sleep? Since I've started dieting, I've noticed I have to go to sleep a bit earlier just because I'm tired (maybe having some caffeine withdrawals too). I'm tired at least an hour earlier than I normally used to going asleep.