It can be due to problem in the neck region /in the shoulder. Please describe the location of the pain and if there is any altered sensation in the arm or fingers/hand.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some strengthening as well as stretching exercises will be helpful for sure.
Health Tips
At home.. Rest +hot water fomentation +pain killer gel +no side lying on same shoulder +no cold exposure +no manual exertions +no weight lifting +no travelling +no jerks to the shoulder. No long sitting.