I have pain on my right shoulder blade during the day and not at night. what could be wrong?

Gravity: The arm and shoulder weigh about 20 pounds, and this weight is borne by muscles (superior trapezium) (st) that suspend this mass when upright. Only connection of shoulder to axial skeleton occurs where collar bone joins to breastbone. Chronic st spasm and strain increases in the upright position. Heat, massage, acupuncture, biofeedback, and physical therapy can help.