This situation may have the following reasons:

After exercise, nerve cells continue to send nerve impulses to muscles, which causes muscles to beat. Muscle nerve self-regulation will produce muscle contraction and beating.

Fatigue can also cause slight muscle spasm, which will be relieved after fatigue is relieved.

I'm afraid that's why you "always feel very tired recently". Have a regular life. It's okay.

How to prevent:

1. Strengthen exercise, improve adaptability, and make full preparations before exercise, especially in cold environment. Massage the spasmodic muscles properly.

2. Keep warm in winter. You can't be inactive in the water for a long time in winter. If the water temperature is low in summer, swimming time should not be too long.

3. Sweat more in summer, pay attention to drink some salt and boiled water to replenish the lost electrolyte.

4. When you are tired, you should not do strenuous exercise for a long time.

If the situation is serious, I suggest you see a doctor and pay attention to protect your health.