Piriformis Syndrome may be the cause... Small muscle in gluteal region. Some percentage of population has sciatic nerve(innervates the back of the leg)going through it...when the small muscle gets inflamed it will expand sometimes impinging the nerve.
A good way to tell if your piriformis is tight is laying down supine on your back, bring your one leg up into a cross-legged stretch, where one knee is placed over the opposite knee, with the foot being brought up to rest on your thigh region. Pull knee towards chest. Hold 30-60sec