This may be possibly due to severe muscle spasm in lower back muscles. Sometimes, it may be associated with some nerve root inflammation or compression. Was there any injury or jerk.
Next Steps
I will suggest you to start with hot fomentation, take complete bed rest for now, take analgesic with muscle relaxant tablet, apply analgesic ointment locally.. If pain doesmt settle down, plan to get one x ray done of lumbosacral spine..
Health Tips
Avoid heavy work, avoid sitting down, avoid Indian toilet..