Muscles very sore, particularly arms, unrelated to exercise. lack energy. tired. what could be reason?

Several possibilitie: Muscle soreness is known as myalgia. This, plus lack of energy, and fatigue could be a virus, which usually resolves in about 3 weeks. Abnormal sleep state, depression could be other causes. There are some muscle disorders such as polymyositis and polymyalgia rheumatica, or some neurologic entities that need be considered if this goes on for more than 3 wks; if so, see your primary care doc.