Disc issues? When pain travels down a leg, especially if it goes below the knee, it is usually from a nerve in the back that is either chemicaly inflamed or mechanically compressed. Lumbar disc problems (bulging or herniated), spinal stenosis, arthritic irritation of the neighboring nerve, can inflame or lean on the nerve causing pain. Injections into these regions (epidural, facets, nerve blocks) can help!
Pinched nerve. Sounds like a pinched nerve in the back, known as radiculopathy; however, it's commonly known as sciatica. If the pain only radiates down the back of the thigh, it could be from arthritis in the weight bearing joints in the lie back; but typically pain from the facet or sacroiliac joints present with a predominance of back pain. You need an eval with your doctor.
Back pain. It sounds like your pain is coming from starting in your low back. Something in your back is causing the nerve that goes to your left leg to get irritated, causing pain. It would be best to be evaluated by your physician to determine the cause.
Sciatica. Pain in the distribution as you suggested describes the pattern of the nerve that travels in the leg called the sciatic nerve. The sciatic nerve is not the cause of the pain source, it is the result of an irritated nerve typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease requiring further evaluation by a spine specialist.