There are many reasons like long sitting very soft or very hard mattress, bad sitting posture ,slouched posture, heavy weight lifting or exertions, sometimes long travelling.
Next Steps
Go to a physiotherapist for proper diagnosis, assessment and treatment and to know the precautions.
Health Tips
For back-at home.hot water fomentation+application of pain killer gel..avoid forward bending, weight lifting, cold exposure , stair climbing.long travelling or driving, no prolonged standing/walking/sitting,use firm mattress.