I agree that you need to take your daughter to the pediatrician for an appointment soon for this issue. There are a couple of different things which came to mind immediately such as dehydration, overheating and over-exertion, low blood sugar, or something with her heart rate. It could be one thing or a combination of things.
Good luck and I do hope it's something that can be managed. I have to pay attention to my blood sugar levels and when I exercise I have to make sure I also stay well hydrated and keep an eye on my body temperature. It's all part of the Fibromyalgia in my case, but that's just me.
A friend has a daughter that has to pay attention to her breathing while exercising. She has severe asthma and severe contact allergies to boot. If she over-exerts herself it kicks in the asthma and she can gag and vomit from having trouble breathing.