I saw a vascular doc for testing. He asked me about pain. I told him I had left butt pain. He poked me with a finger and almost sent me across the room. That's the SI joint he told me. Sitting and lying down were most painful. Walking helped it feel some better. I just had pain management do a SI joint injection and the results have been wonderful. Sleeping on that side does not cause pain! Will have the right side done!