From last 1.5 year I am suffering from low back pain and buttocks pain , I have done  all blood test regarding arthritis and Ankylosing spondylitis My all blood test are quite normal Last year MRI of SI JOINTS and pelvic was quite normal ,in meantime no ESR and CRP increased beyond lab limit ,always come normal, But pain is symmetrical with same intensity and below knee joints also , what other reason possible to the pain ,last month my x ray was showing streghthing of lumbar spine