I'm having pain going down left leg any idea what might be causing this?

Radiating leg pain: There are many causes of pain going down a leg, most often due to sciatica or pinched nerve from spinal pathologies. There are other causes of pain including hip/knee arthritis, vascular conditions, neuropathy, and also nerve entrapment conditions.

Herniated Disk: Pain down the leg is commonly caused by a lumbar herniated disk, especially in a young person. There are other causes, such as hip problems, but the more common thing would be a herniated disk in the back.