I recommend that you see a physiatrist
about this problem. The description of your low back and left leg pain in your question sounds like lumbar radiculopathy. This is the medical term that is sometimes replaced with the term "sciatica", though this really doesn't have to do with the sciatic nerve per se. Most of the time this pain comes from degenerative disc disease in the low back (arthritis
of the vertebrae) which can result in either foramina stenosis (narrowing of the nerve exits from the spinal canal) or spinal stenosis (narrowing of the spinal canal). When this type of problem arises, I usually start with the patient undergoing a round of physical therapy. This often results in improvement and no imaging is needed. If the pain continues, then I will often next get an MRI
of the lumbar spine. This will show if there is a pinched nerve somewhere. If a pinched nerve is present, then it might be fixable with either further physical therapy, time, injections of steroids into the low back, or in some cases surgery. The best physician for you to see about this problem is a physiatrist. This is a physician that specializes in physical medicine and sees patients with back pain. They are usually trained in performing the injections that are sometimes needed to relieve lumbar radiculopathy.