Its because of the lack of muscular strength of your back which generally occurs due to overuse of muscles. Consult a physiotherapist.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +Heating therapy +some strengthening 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 +correct posture (sit tall).