It can be due to frozen shoulder in which muscle and capsule of the joint becomes tight and do not allow the movements of the shoulder above head, back and creates pain specially at night, small and simple daily activities become painful. Diabetics are prone of this condition.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some strengthening and 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 jerks to the shoulder.