Hi, As you are not obese, no history of back pain, no history of injuries either and physically fit as well this symptoms of the pain and relieving it directs me towards the nerve entrapment. Since it is understood that during lying in supine position there is a strain on what lumbar spine do the normal curvature of the lordosis will be having a strain on the nerve root via the strain of the para spinal muscles leading to nerve compression and pain over the hip. As soon as the pain starts you take the prone position by which the strain on the paraspinal muscles and lumbar spine is released and the relief is achieved. This is my understanding with respect to the history provided. If you don t mind than you can have a physical examination of the same by the orthopedist and this can be taken further with simple exercises of the core stability. n my clinical practice of over 12 years, majority of the pain aggravating and relieving are exactly as you mentioned so I did suspect for paraspinal muscle spasm. Most cases recover with core stability exercise alone. Hope I have answered your query. Let me know if I can assist you further. Regards, Jay Indravadan Patel, Physical Therapist or Physiotherapist