You seem to be having WEAKNESS OF LUMBAR MUSCLES. COMPUTER FATIGUE
SYNDROME may be the cause.
It means that your back muscles are weak. You need to do certain exercises to strengthen them. A qualified physiotherapist may teach you such exercises.
Working on computer for a prolonged period may lead to such a problem.
You need to follow the RULE OF 20 while working on a computer.
After every 20 minutes, see at a distance of 20 feet for 20 seconds and walk for a distance of 20 paces.