Hi D.,
It definately sounds like hypoglycemia. And, just an fyi, hypoglycemia is rarely diagnosed - the tests for it are so dependant on the blood content at the moment of the drop that it is hard to catch. The really good news is that all it takes is a little education on what causes drops and how to resolve them to really help your daughter.
First, I would recommend doing some reading so that you understand the physical nature of it and what foods are good for resolving it - not all foods are good, some cause shorter relief then others.
Second, just watch your kiddo and find out for yourself what works for her. All kids are different. For my son, who inherited his hypoglycemia from his dad and I, we find that he needs a both protien and a fruit at every meal to stay balanced. He has three large meals and is allowed snacks throughout the day. He has one large snack before bed that alwasy includes a protien like a peanut butter sandwich, cottage cheese or yogurt.
We have found a balance for his particular phsyiology that keeps him happy, and we are always responsive when we see his mood changing. He bottoms out pretty rarely these days. But, sure enough, if he starts getting frusrated or acting out, something as little as a cheese stick can make it a lot better.
Good luck. The great news is that it is easy to treat on your own with some eduction and figuring out what works best for your kiddo. After awhile, you won't even think about it, it will just be second nature to see the signs and treat them.
Have a great holiday and a happy new year!!!
D.