I was wondering if anyone else has experienced this. I had a spinal fusion and a replacement disk between L5-S1 February 2015. Yesterday I noticed while sitting and flexing my buttock muscles it felt like my tailbone was shifting. I didn't have any pain, it just felt weird. Today it has been shifting every time I move while sitting. Ive noticed some sciatic pain but not much. My lower back has been aching slightly, but nothing drastic. I'm calling my dr next week but just wondered if this has happened to anyone else.