How frightening for you. It doesn't sound like you overreacted at all.
My son has fasting hypoglycaemia. It's part of his growth condition. It is supposed to improve as he gets older. We have a blood sugar monitoring kit and when he's ill and therefore not eating, feed constantly (as in every 20 mins) with dioralyte with added fructose, more often if it's a tummy bug. Through the night as well, via his gastrostomy.
I would definitely get a home blood sugar testing kit. Also worth knowing that if she has another episode and is very drowsy, you can rub jam onto the inside of her cheek. Some of the sugar is absorbed, and you don't run the risk of choking. You can get glucose gel pouches as well which I think can be used in the same way.
I hope your DD is investigated soon, and you have some answers.