From your description it appears to be muscular back pain. Majority of this is caused by incorrect sitting posture, weak core strength and incorrect carrying of loads. If there is no pain going down the back of legs or no numbness, this most likely represents musculoskeletal back pain.
Next Steps
Work on keeping correct posture while sitting. Work on core strength with core stability exercises or yoga.
Health Tips
Seek medical opinion if symptoms do not improve or if any numbness develops.