WHY DO I KEEP ON GETTING BLACK OUTS?

I think you should get a new doctor. You are underweight. 5'8" and 112 lbs is too light. You've been watching too many TV commercials and all the skinny models. It's true you will eventually gain weight as you grow older but at the rate you're eating you may not get there. See a nutrition expert for a proper diet. When you get the energy back then you can exercise and be "normal" again. Good Luck.