Its because of the lack of muscular strength of your back which generally occurs due to overuse which time and causes pain after a long use. Shoulder pain can be due to cervical problem or due to acute frozen shoulder.
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 long sitting /standing /travelling /driving /walking +no stair climbing +no cold exposure +no manual exertions +no weight lifting +no jerks to the shoulder +no neck /back forward bending +sit with back rest/cushion support +correct posture (sit tall).