What could be the cause of my pain in my low left back it runs to my buttocks ang leg?

Piriformis Syndrome!: I would bet on this, but radiculopathy still possible. See a rheumatologist or physical medicine doctor to help figure this out. The pirformis syndrome can be diagnosed and treated by a successful injection!

Sciatica?: You did not give much to go on, but i would guess sciatica. With that said, i would see a doctor to be sure it's not something more dangerous. If it is sciatica physical therapy or yoga may help.