It could be referred pain, from the SI joint or from elsewhere.
Ginny - 56 year old female
Dx with USpA in March 2013; changed to AS in July 2015
Iritis and Scleritis
unicompartmental knee replacement June 2014
MTX, Humira, Cyclobenzaprine, plus Indomethacin ER as needed
Supplements: Folic Acid, Vitamin D, Calcium, Fish Oil, Melatonin, Culturelle probiotic