When you start fitness, you will consume water and some fat in your body, so you will feel that your weight is falling. When fitness continues, your body will break down fat and turn it into muscle. Muscle is heavier than fat, but its volume is much smaller than fat, so you will have a small belly on the outside. In fact, your weight will be slightly heavier with the increase of muscles. Don't worry, keep exercising, your body will get better and better, and your weight will stay within the normal range.