hot flushes are very rare in children. fainting can be due to simple causes, but rarely to some heart problem.
I would go to see the pediatrician
Is she on any meds? as some have side effects if she is ask if she needs the dosage adjusting, or the med changing. Is she drinking enough water especially if she is exercising and in hot weather make sure she is well hydrated as heat stroke can have the symptoms you describe .Good Luck let us know how she is doing