I Am Having Pain In My Lower Left Buttock Upon

I am having pain in my lower left buttock upon standing for more than 15 minutes accompanied by numbness on the outside aspect of my left thigh. This is usually relieved by sitting, or walking at a somewhat brisk pace. I had a lumbar laminectomy - L4-L5 33 years ago. I am currently overweight but am losing at a slow pace. thank you for listening