Exhaustion is totally normal in the beginning. Your body is adjusting to functioning on a much lower caloric intake and it takes some time to balance out. I think my energy kicked in around 3-4 weeks (which is also why they tell you not to start new exercise regimen the first 3 weeks).
On a side note, I'm about 8 weeks in and have realized that I sleep like a baby. Seriously, I haven't slept this well in 18 years (before kids). I used to have to read for 1-2 hous to fall asleep at night and then I'd wake up several times in the night too. Now I'm lucky if I can finish 3 pages before I'm out, and I'm not waking until the alam goes off. It's awesome!
Every day is a battle, but whatever, I'm scrappy ~Darcey