Hi sakerwalla,
Your last post did it for me...it sounds like a thorax dysfunction causing myofascial referred pain.
The 'pop' doesn't sound that serious.
She was nearly finished in the race - she only had 5 more miles to go so was 80% through the race - fatigue is likely and she probably went into a habitual motor pattern and likely overused her obliques to help her breathing or stability (i don't think her core stability is that good if the belt helps her BTW)
The thorax experts are definitely LJ and Diane Lee, LJ esp in the rehab side of things.
Good luck with her - theSIJ is likely to be secondary to the thorax giving way.
BTW, thanks for the good posts sharileedahl!