The pain may be possibly due to sudden spasm in lower back muscles.. it may be associated with some bad posture for long time..
Next Steps
Please start with hot fomentation, analgesic with muscle relaxant, apply analgesic ointment locally.. If the pain doesn't settle down, you may require ti get one x ray of Lumbosacral spine..
Health Tips
Avoid heavy work, avoid sitting down, avoid Indian toilet, avoid forward bending...