My right shoulder muscle is painful what could be the cause ?

Shoulder: Usually if it is one sided shoulder pain either a mild strain of the muscle and tendon can occur or if you lie on that side while sleeping for a long period, there is mild strain. The shoulder tendon and muscle groups are very complex and help move your arm in many directions so local ice and/or heat, mild stretching helps to gradually get it back to normal. If not, see your doctor.