Hi Sarah,
Its all well and good releasing the sacroliliac joint, but unless you strengthen around the area to keep it in that position, then as it is doing, it will just revert back to causing you pain. Your opposite side will be working overtime to take some of the pressure off the injured area, which is why the muscles won't release.
I would initially strengthen the abdominals with pelvic floor type exercises, then stengthen Glut Med for stability.