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.