Muscle pain is most frequently related to tension, overuse, or muscle injury from exercise or physically-demanding work. In these situations, the pain tends to involve specific muscles and starts during or just after the activity. It is usually obvious which activity is causing the pain.
Muscle pain also can be a sign of conditions affecting your whole body, like some infections (including the flu) and disorders that affect connective tissues throughout the body (such as lupus).
One common cause of muscle aches and pain is fibromyalgia, a condition that includes tenderness in your muscles and surrounding soft tissue, sleep difficulties, fatigue, and headaches.