What Causes A Constant Pain On The Shoulder And Arm?

I just came from my doctor's office to ask about the constant "ache" in my right shoulder/arm. (Worse at night!) She suggests it is probably inflammation of the 'trapezius muscle." Was given Meloxicam and tiZANidine prescriptions and told to return for exrays a month from now (because we had planned a long car trip) and if no relief from these anti-inflammatory meds.