I've been consuming 500 calories for the past 12 weeks (lost 36 lbs)but i fainted! and have no energy. how can i up my calories without gaining weight

Too Much Weight loss: First congratulations on your dedication. But, you may be killing yourself. It may be wise to add a protein bar or a protein shake to your current regimen and get to a place of about 1200 kcals. You need nutrition. You can get this with some veggies and some protein. You do not need to gain weight but you do need to eat.

Increase your intake: Extremely low calories are not healthy and may deprive your body of adequate nutrients. At mercy we recommend low calorie diet of not less than 800calories. Calorie reduction is not enough to lose and maintain. Add exercise and select foods high in fiber, protein and complex low glycemic index carbonhydrates.