I know how miserable that can be I had similar "spells".  They started when I was about 23, but they included diahrrea, and I didn't start passing out with them till I was older, early 30's.  My doctor diagnosed me with something called the vega effect which was made worse when I got hypoglycemic.  The vega nerve controls a lot of our bodily functions and is separate (outside) our spinal column.  The urge to go to the bathroom would travel down this nerve so fast it would hit bottom and then bounce back up the nerve.  This would cause heart rate and blood pressure to fall drastically.  It caused the same symptoms as your grand-daughters except I had the diahrrea.  She might want to ask her doctor about it.