Dear Stella,
I am suffering from exactly the same pain. I have had MRI, nerve conduction study, CT scan etc etc. The Doctors (I consulted a lot of them) have ruled out slipped disc, arterial blocks/diseases, broken bones etc etc. They just don't know the cause of the pain yet. Virtually I have tested myself for every other condition/disease and it turns out I am perfectly normal. A full body MR even checked for blocks in my entire arterial system and nothing has been found. The doctors declared that I am hale and hearty and I have no cause for worry of any sort.
Just that the pain does not go away. It starts from the left buttock (gluteal) and just moves down towards the side of the thigh. Even my positioning and leg movements have changed because of the pain.
Have you tried contacting a sports medicine practitioner because it may be a simple case of sciatica or piriformus stretch.
Since I do not have a sports medicine physician located nearby, I am turning to myself for help. I am going to take rest for 21 days and am going to a massage clinic for some deep massage (for the next 21 days). Also I have prescribed myself lots of water (at least 3 quarts/day) for the next fortnight after which I will be bringing it down to 2 quarts / day. Strectching seems to relieve my pain. So I am drawing up a chart to strech my thigh and buttock muscles (consult your local physiotherapist). Lets see if I am cured by this time next month. Will keep you posted
its probably herniated disk putting pressure on an outlet nerve(sciatica) if i were your physician and depending on your insurance i would order an MRI just to make sure...look up herniated lumbar disk on google it should decribe your exact symptoms