Its because of trapezius muscle weakness of cervical region. You should consult a physiotherapist..
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 /forward neck bending /travelling /driving /computer work/table work /studies +no cold exposure +no manual exertions +no weight lifting +postural correction (sit tall).