Weakness is a feeling of being tired or exhausted, or experiencing a loss of strength. Weakness may not always be accompanied by obvious or visible illness. Short-term weakness may occur because of overwork, stress, or lack of sleep. You may also feel weakness after overcoming an illness, such as a cold or the flu. Some weakness may occur after vigorous physical activity.Weakness may occur throughout your entire body or in a specific area, such as your arms or legs. Weakness may also be localized to a single muscle such as a calf muscle in your leg.