I've had this problem for years. It's very painful. I'm very self conscious a out it. I ca t lay flat on my back due to my coccyx. It feels like a hard ball of bone right above my butt crack. I was told I have sacroliac joint dysfunction and that the ligaments fused together and that's why it feels that way, then I was told degenerative disk disease, but the pain management doctor said it's osteo arthritis. I have no idea. All I know is it is very weird looking and u can feel a big ball of bone in my lower back. Does anyone else have this problem or have any other ideas what it could be?