What causes pain in the upper back?

Muscle spasm : When the body center of gravity is off center due to pelvic girdle instability, the body tower tilts. Upper body muscles are exposed to extra burden of holding upper body centrally. The muscles of the upper back inherit this job. Over time they become spastic. Pain is often found at the top of the shoulder blade and at the medial border of the scapula, where the rhomboid muscles attach.