Answer

Hello Customer!
This is usually musculoskeletal and the radiation down the leg is due to nerve impingment from swelling or pressure. Lateral femoral cutaneous nerve syndrome is an entrapment at the hip area superficially and can cause radiation down the outside of the leg to the uppper knee. This is usually from pressure (seen a lot in men for belts or pants being too tight at the area. Taking pressure off the outer hip if this is an issue resolves it in time. If you are a runner IT band syndrome can cause this. This is an inflammation of the fascial type muscle over the hip and outer thigh. Other muscle strains or hip bursitis can also create thse symptoms.
The good news is as long as you have no fever or loss of function this is usually treated the same way initially for all of the above. Ice to the area with rest. Over the counter motrin or ibuprofen 600-800mg every 6 hours or so for up to a week. Gentle stretching by laying on your side and stretching the affected leg over the other to feel a stretch, but not pain in the area.
If no response after a week then I would see your dr to rule out other more serious musculoskeletal problems and the possible need for a prescription treatment.
Good Luck
Hope this helps!