This pain can be due to 2 reasons.. It can be due to problem in the cervical area leading to nerve compression and trapezius muscle pain.. Or it can be due to problem in the shoulder region. Is it more painful at night, not able to move the hand behind your back, no able to move overhead? If yes, it can be a frozen shoulder... Best is to consult a physiotherapist for proper assessment, diagnosis and treatment.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy, TENS, heat therapy and a few exercises will be helpful.
Health Tips
At home.. Do some hot water bottle fomentation and apply some painkiller gel, avoid cold exposure and avoid side lying on the same side of pain.