When you get off carbs or even just fasting the body is still used to get carbs or some food from the outside. Your body has not adapted yet and not learned to use the stored fat for energy, this can take days maybe weeks, I am also waiting for a energy boost so many people talk about, I did 40h fasts and also feel weak on the fasting days, no chance to workout, sometimes even hard to just stand after getting up, but that’s the way it is.