Difficult to say without examination.
Possibilities that come to mind can include a pinched nerve, cervical disk disease, rheumatological disease (like polymyalgia rheumatica) and muscle spasm or strain.
Workup can include imaging (like plain films of the neck or an MRI, and a rheumatological workup (including ESR and ANA levels).
A referral to physical therapy can be considered if a muscle spasm or strain is considered.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.