I always felt pain on the upper back(left side) if i sat or stand too long. what cause this? i felt it almost everyday

Most likely muscular: Most common is poor posture. If you don't sit properly over time this can cause fatigue of your muscles. There is also a condition called myofascial pain syndrome, wherein there are areas of "trigger points" along a specific muscle group. Lastly, arthritis conditions of your thoracic spine needs to be ruled out. All of this could be evaluated by your physician.