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Was there an injury? or lifting a heavy object ? How & When did the pain start?
Give your arm rest , fomentation with hot water bottle helps.
If its a sprain
or such, it wil get better by doing above mentioned , in a day or 2. But if its something else (ex- nerve
compression ) then treatment may not be as simple. So you will have to visit a orthopedician, an x-ray of the neck will be required, and ofcourse physiotherapy