There are many reasons for low back pain.. Most common cause is acute lower back muscle spasm which might have happened in your case..
Next Steps
You can start with hot fomentation, take analgesic with muscle relaxant tablet, apply analgesic ointment locally.. If the pain doesn't settle down, you may require to get one x ray done of lumbo sacral spine..
Health Tips
Avoid heavy work, avoid sitting down, avoid forward bending..