Thanks for the question.
To diagnose this, you need an immediate orthopedic exam.
It could be:
Pinched nerve in the shoulder.
Pinched nerve in the neck.
Old injury in the shoulder.
Muscle disease. [unlikely].
Arthritis in the shoulder.
In most cases the treatment is going to be the correct physical therapy.
You can use ice or heat for now, until you see the doc, whichever helps more.
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