The sacroiliac (SI) joint is where your pelvis (the ilium) meets your tailbone (the sacrum). Inflammation or other problems in the SI joint can cause pain up through your low back. Dysfunction means something isn't working right. In the case of the SI joint, dysfunction has to do with the alignment and movement of the joint. When alignment has changed, or the joint isn't moving right, ligaments around the SI may be stretched to the point that you feel pain.