This treatment is being suggested on bases of the information provided. However I would like to examine & investigate you  in detail. -Control  Hypertension, Diabetes or  metabolic disorder, if there is any. --. Sleep on a hard bed with soft bedding. --. Keep no pillow under the head. --. Avoid painful acts & activities. --. Fomentation with warm water. Let the part not be exposed to cold. -- .Do mild exercises for the affected part.- neck and shoulders. --.(Take help of physiotherapist or you can visit my YouTube channel Dr. N. C. Gupta and watch 'Me & My Yoga' and 'Yoga Simplified'. https://youtube.com/channel/UCPEQc6B1ywzwin048JSu6rw --Do not ignore, let it not become beginning of a major problem. -Any way it may be tried, --. Ugesic (Piroxicam) 20mg 1 tab od x 5days --. Caldikind plus  (Mankind) 1 tab OD x 10 days. -(You may need help of your local doctor to get these medicines.) --Do ask for a detailed treatment plan. Kindly make sure, there is no allergy to any of these medicines. --For emergency treatment visit nearest hospital. --Wish you a quick recovery & good health.  Dr N C Gupta MS (ortho)

Reasons can be many Consult online as more info is required

Can be due to many things- muscle spasm due to overuse or improper posture, arthritis, rotator cuff tendinitis or tear, biceps tendon tendinitis or tear, frozen shoulder being the more common reasons.