A slipped disc can cause back pain and if it is pressing on a nerve then the pain can come to ur legs or buttocks. In your case the small joint which connects two nearby vertebrae (bones that make the whole backbone) has developed arthritis (joint is damaged) which is the cause of back pain predominantly.
Next Steps
A combination of medicines, physiotherapy and lifestyle modifications would help.
Health Tips
Avoid lifting weights or activities that need bending forwards.