Most common cause is excessive use of the trapezius muscle on the affected side.
Sleeping on the same side or typing or heavy lifting or long driving can cause this problem usually.
The muscle gets fatigued lactic acid accumulates and causes achiness.
Dehydration, lack of magnesium and potassium also can precipitate this.
Not serious since no other symptoms like radiating pain to upper extremity o towards the back of head, no limb weakness either.
Please drink plenty of fluids, warm compress, neck massage and Naproxen can be tried for symptomatic relief.
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