Hello
As per your question the reason of your back pain is long sitting hours and tightness in muscles.when muscle remain tight for long they even loose their strength thus causing uneasiness and pain that slowly becomes constant.
Next Steps
start doing stretching exercises for back muscles and lower limbs.
Stretching will help in improving flexibility thus reducing pain.
For further evaluation plz contact a good physio nearby.
Health Tips
consume 3-4 L of fluid everyday.
avoid sitting for more than 1 hr.