If you ate only 1000 calories a day and lost 15 pounds, you probobly lost muscle mass. The body doesn't really like to burn fat. Add starvation mode and the body burns muscle proteins instead of fat since muscle is protein and a source of energy. Now, given that you lost musclemass, you now need less calories to maintain a bodyweight. In other words, your metabolism slowed down. This is why a 200 lbs guy needs to eat more than a 130 lbs guy to maintain his weight.
So, your body is in starvationmode by now. When you go from eating 1000 calories a day to eating high calories without gradually upping the dayly calorie intake (300-500 a week), the body responds by spiking insuline and stores the calories in the fat celles. Also if you ate alot of carbs, they draw water and glucose into your muscles, wich also weighs a bit. A fair amount of that weight you lost might be waterweight. But still, if you lose more than 2 lbs a week no matter what diet you're on, most of it is muscle. Unless youre morbidly obece ofcource.
