1. Muscle growth: When you start fitness, your body begins to produce new muscles. Muscle is heavier than fat, so if you gain weight, but most of the increase is muscle, then you may feel "fat", but in fact your body fat rate may have dropped.

2. Diet problem: After fitness, you may eat more than usual, because your body needs more energy to repair and build new muscles. If your diet doesn't increase accordingly, you may gain weight.

3. Excessive water intake: Fitness will make you sweat, which may lead to excessive water intake. If this extra water is not effectively used by the body, it may be stored in the form of weight.

4. Stress and anxiety: Fitness can release endorphins, which are chemicals that can help you feel happy and satisfied. However, if you feel stressed or anxious after fitness, you may overeat to try to relieve these emotions.

5. Insufficient recovery: If you often do high-intensity exercise, your body may not have enough time to recover and repair. This may cause you to feel tired and weak for a few days after exercise, thus affecting your appetite and eating habits.