Sacroiliac joints can be a source of pain but rarely are painful. There is a disorder called spondyloarthropathy that can cause this pain. Look at “slideshare” on the internet as I had placed a lecture regarding this disorder on that service.
I have seen one sacral stress fracture in the past but obviously they are rare. An MRI will diagnose this injuries.
The most common source of this pain is most likely degenerative disc or facet disease.
Donald Corenman, MD, DC is a highly-regarded spine surgeon, considered an expert in the area of neck and back pain. Trained as both a Medical Doctor and Doctor of Chiropractic, Dr. Corenman earned academic appointments as Clinical Assistant Professor and Assistant Professor of Orthopaedic Surgery at the University of Colorado Health Sciences Center, and his research on spine surgery and rehabilitation has resulted in the publication of multiple peer-reviewed articles and two books.