Sorry for late reply and inconvenience.
Thanks for all this additional information.
I understand your concern.
Well as you describe, it sounds like sprain/ Tendonitis.
The symptoms for a few months is worrisome. I would suggest you have a physical check-up along with few investigations like MRI to clinch the diagnosis. Treatment will follow.
Till then give rest to the shoulder/ avoid postures which hurt/ avoid sleeping on the arm concerned/ anti-inflammatory tabs like Motrin and cold compress may help.
Another possible cause may include Pinched nerve and rotator cuff strain.
See an Orthopedist.
Consider Physical therapy.
Let me know if you have any further query so we can discuss it thoroughly in every aspect.
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