You're probably burning about 1600 calories a day without exercise - ie that's the amount your body uses to keep your organs running and your cells regenerating. With strenuous exercising and barely any food that's a massive daily calorie deficit. Sounds good on paper, but your body's response to having not nearly enough food is to clamp down, lower your metabolism, and start using your muscle for food rather than your fat. It's a smart physiological response if you're starving in a desert, but fucks you over completely if you're trying to reduce fat.