A sudden drop in blood pressure, or blood
sugar, could cause fainting. Sudden bleeding internally can cause blood
pressure to drop suddenly. I'd deduce that something burst or ruptured that caused the bleeding, like a small vein, or cluster of capillaries, or a cyst. There's also parasites that can get up in the bladder, like pinworms, which are very, very common in gradeschool.
I suppose you could also consider that she may have passed a kidney stone or bladder stone.