Lower back pain on left side?

Probably your muscle: Usually lower back pain is due to muscle strain that should improve with gently stretching the back, giving the muscle some rest (but remain active if you can!), and using an over the counter pain reliever / anti-inflammatory. If the pain becomes severe or does not start to improve, or if it's accompanied by other symptoms, you should be evaluated by a doctor.

Numerous possible: Pain in that area could be due to a trigger point (localized soft tissue inflammation) vs muscle sprain/strain vs facet joint pain vs degenerative disc vs disc herniation, among other causes. If your symptoms are persistent or worsening, see a physician.