If you lost 15 pounds in a week, you lost too much. Are you not
counting calories, along with the carbs? Your body needs certain foods, in adequate amounts, in order to be healthy and you need a certain number of calories/day just to stay alive.
Are you getting any exercise? If you aren't exercising to keep your muscles strong, you could be losing lean muscle, not fat, which is going to harm you in the long run.