Lwhat causes lower back pain on your lower left side?

Multip possibilities. Low back pain can be caused by various conditions and often a combination of sorts. The history (due to trauma?/how long present/ etc) , severity of the pain, radiation of the pain(does it move down your leg) & other factors, and exam are helpful in identifying the cause. It coud be be due to mechanical prob, muscle strains, disc disease, arthritic problems, joint d/o. See your md as a start!
Musculoskeletal. If you have a relatively small region of your back that hurts you, it can be from the si joint (sacroiliac), lower lumbar joints, muscle spasms, ligament or tendon inflammation. These are the most common reasons for regional back pain. Try conservative treatments like ice/heat, otcs like ibuprofen-type products. Stretching exercises. If doesn't go away, go to doc for exam and options.