Treatment with diagnosis is exactly like shooting in the dark. Chances are you are going to miss, most of the time. Consult an orthopedist and get the necessary imaging done. Shoulder is a complex joint, what you are experiencing can happen due to a lot of reasons and variety of conditions.
Next Steps
Consult an orthopedist.
Health Tips
In the meantime you can apply ice for 15 mins. Followed by hot waterbag application for 15 mins. Treatment area should also include the scapulae. Also give rest to your shoulder.