1. Your body may have adapted to a state of inactivity. After starting fitness, your body needs some time to adapt to the new amount of exercise, which may lead to weight gain.
If you increase your food intake after fitness, it may also lead to weight gain.
When you are exercising, your body needs more energy to support the exercise. If you consume more energy than you do, you may gain weight.
In addition, clothes and pants are tight, probably because your muscles are growing. In the process of fitness, your muscles will gradually become stronger and stronger, which may lead to changes in body shape and make clothes and pants feel tighter.
Generally speaking, the change of weight is not always reflected in your appearance, because muscles are tighter than fat. So, even if your weight doesn't change significantly, your pants may still get tight. This is because your body is adapting to the new amount of exercise, and it may take some time to adapt to this change.