What Causes Fatigue And Muscle Weakness?

I am 27 white male. Fairly healthy, smoker, overweight by 15-20lbs. Fairly active lifestyle. Eat fairly healthy. Woke one morning with sudden fatigue to muscles of my entire body. Some discomfort but not painful. Muscle weakness to the point of not even being able to turn a door knob, or fastening my jeans. Sudden onset and after four days feel only slightly better. No other symptoms such as headache, fever, rash. No sinus problems or lymph nodes swollen. Only symptom is fatigued slightly uncomfortable muscles. So much fatigue cannot function normally.