Most likely it is acute back pain/paraspinal muscle spasm due to prolonged sitting and poor posture. Local tenderness without trauma suggests muscular origin rather than spine injury.
Next Steps
Tab Aceclofenac + Paracetamol twice daily after food × 5 days
Tab Thiocolchicoside at night × 3 days
Local Diclofenac gel massage 3 times daily
Hot fomentation 15 min, 3 times/day
Avoid prolonged sitting
start gentle back stretching after pain reduces
Health Tips
Consult immediately if radiating leg pain, numbness, weakness, fever, night pain, or bladder/bowel symptoms develops.