Could be not enough fat. Could be not enough calories. Could be your body hasn't kicked into ketosis yet. Could be it's not the right diet for you.

It's really common to feel sick at the beginning of the diet, because you're body isn't used to burning fat for fuel. I can't remember anyone complaining of weakness, though. It usually actually sounds like flu symptoms.

http://elowcarbfoodlist.org/the-keto...ms-and-relief/

I personally never had it, but then I never got the burst of energy either. I think they go together. You feel awful for a week, then you feel great.