What might be wrong with me if my lower back on my left side is sore and my left leg goes numb?

Sciatica : Sciatica or spinal stenosis.Sciatica refers to pain that radiates along the path of the sciatic nerve, which branches from your lower back through your hips and buttocks and down each leg. Typically, sciatica affects only one side of your body. Sciatica most commonly occurs when a herniated disk, bone spur on the spine or narrowing of the spine (spinal stenosis) compresses part of the nerve.