Hi Joanne,
This is something I don't know the answer to because I don't know the odds between one condition and the other, but I found a couple of informative links that briefly cover other problems that can cause sacroiliac joint pain.

http://www.medicinenet.com/sacroiliac_joint_pain/article.htm
http://www.medicinenet.com/buttock_pain/symptoms.htm

Here are some tips for "Sacroiliac Joint Dysfunction" pain.
http://www.lower-back-pain-answers.com/sacroiliac-joint-dysfunction.html

From what little I understand about spondyloarthropathies in general, is that they all have the potential to cause back pain, and quite possibly, sacroiliac joint pain too.

HLA-B27+, JRA diagnosis in 1981, re-diagnosed as AS in 1988. Also iritis, colitis, and psoriasis. NSD + low carb helps me. My health makes it hard for me to post in a timely way.