That's because you exercise too much or don't exercise for a long time. This is called delayed muscle soreness.

The pain peaked 24-72 hours after exercise and basically disappeared after 5-7 days. In addition to soreness, there are muscle stiffness, mild tenderness and severe muscle swelling, which hinder activities. Delayed muscle soreness may occur in any skeletal muscle after strenuous exercise, especially after long-distance running. Long-distance runners will have pain in the extensor and flexor muscles of their hips, thighs and calves, especially at the distal end of the joint between muscles and tendons. It is suggested to adjust the amount of exercise to moderate physical reaction.