On and off for the past several days I've been having this aching sensation in my pelvis and low back. It usually starts around the top of my hipbones and eventually moves down into my SI joints, where it then starts to move up my spine. It's kind of a dull, burning ache, but it's really persistent. It hurts most when I'm sitting down. When I'm standing or lying down its not as much of a problem. I have a history of back pain and sprained my SI about four years ago. It continues to be painful.