Are you diabetic? If yes it may be diabetic radiculopathy if not. Then see if it is trapezitis or facet joint distinction.
If still not conclusive and after all possible differential diagnosis,when the reason is not know then it is called idiopathic pain.
Instead of trying to identifying the reason go for symptomatic treatment like tens, nerve block etc.
Next Steps
meet doctors at pain clinic
Health Tips
complete blood analysis, alternative therapy