There is a possibility of nerve compression in lower back region associated with muscle spasm. This may be causing the problem.
The problem might have started due to continuous load over the back region and due to over weight..
Next Steps
I will suggest you to start with hot fomentation, analgesic with muscle relaxant tablet, apply analgesic ointment locally. Plan to get X ray of Lumbosacral spine if there is no relief at all.
Health Tips
Avoid heavy work, avoid sitting down, avoid Indian toilet, avoid forward bending..