Super Lethargic...normal?

This is my second time fasting and I'm three days into it. I'm basically over the feeling of hunger, but when I woke up and stood up this morning I was light headed, and really lethargic. The same feeling came over me during my first fast three days into it, but I had to stop because I was going back to school and figured there was no way I could do school half asleep. So I stopped four days into it. With summer here I have no interfering schedule but if I stay as lethargic as I am I won't be able to get the few things done that I need to do (fixing cars amoung other things around the house). Will this feeling pass? Am I supposed to feel and energy boost? At 18, am I too young to be doing this? I developed a little gut, and figured fasting would also remove any of the toxins in my body as well as remove the gut....or am I just a fool. Any insight is much appreciated.
Thanx,
Luke