Very normal and will pass. Usually within a week or so.
All you can really do is accept it and go with it. Relax as much as you can, drink your two litres and just take it easy.
I've just started week 7 and have felt great - once I got into week 2. Only just starting to feel lousy after an almost 2 stone loss taking me to 10 st 7 do I'm going to start working up the steps now for another half a stone or so.
I think it's great for a short ish term weight loss - just take it easy and work up the steps if you are still feeling awful next week
How much would you like to lose?