First, you haven't exercised for a long time. Today, you suddenly exercised violently and your body didn't adapt.

Second, I haven't exercised much, and my physical fitness and muscles have been at a low level, so I can't adapt;

Third, the least likely thing is that you didn't eat well or have a bad rest, and your physical strength is insufficient;

Fourth, you may weigh a lot, such as more than 200 kilograms. Squats without weight bring more load to muscles than ordinary people.

It may be one or several of these reasons.