Why do I feel weak some days?

Fatigue may result from overwork, poor sleep, worry, boredom, or lack of exercise. It is a symptom that may be caused by illness, medicine, or medical treatment such as chemotherapy. Anxiety or depression can also cause fatigue.

Why does my body feel heavy when I exercise?

Two factors make your muscles feel heavy and tired: a lack of stored muscle sugar, known as glycogen, which fuels muscles during exercise, and damage to microscopic muscle fibers. Muscles fill up with glycogen much faster when they are rested than when they are casually exercised.