Had the same thing. The early weeks were like that for me (apathy, tiredness) and slowly got better after that.
There's actually a physiological reason for it, according to what I've read on the internet: Alcohol increases the flow of dopamine in the brain, so the brain stops having to do it by itself. When we stop drinking, we're left with a lack of that chemical, which is related to mood. In other words, we don't have any "feel good" chemical production and it takes some time for the brain to start up again.
I'm sure that's an oversimplification of it, but it's nice to know that it won't last forever. If it gets to be depression, it's better to get some help, though.