I dont have any official sources, but here's what i've gathered about the body in a low intake situation:
When you go from eating 2000-4000 calories/day to 500, your body assumes that there is a widespread lack of food available and goes into a form of conservation hibernation, which prevents your body from breaking down the Fat cells to make energy, instead your body will start to cannibalize and start to digest muscle and organs in order to have enough energy to perform.
The key to weight loss is getting the metabolism up so that you are nearly constantly burning calories (eat small meals 5-7 times a day). Once your metabolism is high enough, your body will begin to breakdown the fat when you're not eating in order to maintain its metabolic rate. While you exercise, you build more muscle, which then uses the glucose put into the blood when fat is broken down into energy. The more muscle you have, the more glucose is used, the more your body will break down fat into energy.
However, it appears as though your body has already entered the starvation/conservation/hibernation phase, and you'll need to help wake it up again. To do this, start increasing your caloric intake to a normal rate; dont JUMP from 500 to 4000 calories/day, gradually increase your intake by 500/day until you're at 2000 NET calories/day (net calories is the total number of calories you eat minus the number of calories you burn through exercise: Total-Exercise=Net).
I'm no doctor, but here's what i recommend: Once you have weaned yourself back onto food, and your body has stopped hibernating; Stay at 2000 NET calories/day for 2 weeks, only start exercising again when you get back to 2000 net calories/day, try to even-out your output/intake so its a consistent daily exercise and caloric intake.
If at the end of 2 weeks of no LESS than 1600 Net calories/day, you have not LOST FAT weight you should see a doctor for some blood tests to see if there is something more serious going on.
Just my $0.02