--as if you may have some compression going on in the sicatic nerve distribution or its branches--it can come from compression on the nerve (the body's largest) itself, of from spinal compression "higher up".

Most people with metabolic, vascular, or autoimmune neuropathies seem vulnerable to the "double crush effect"--that is, once a nerve has been damaged, an additional compressive insult produces symptoms that are more than the sum of the constituent parts, so to speak. It may be worth it to get a little imaging done.