Its a muscular pain which occurs due to overuse of back muscles and increases with immobilization and when lumbar spine becomes flat.
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 /walking /travelling /driving /computer work /table work /stair climbing +no cold exposure +no manual exertions +no weight lifting +correct posture.