I Am A Nurse And I Work The First Aid

I am a nurse and I work the first aid for high school show choir. I have a 17 yr old that is overweight but performs at high levels in the competitive show choir. They perform vigorously for 20 minutes and then suddenly stop. She faints after every performance and is very somnolent and takes her every bit of an hour at an almost unconscious state to recover enough to walk and then still seems fatigued and sluggish. She does not seem overheated but we put ice packs on her anyway, does not have trouble breathing, we checked her blood sugar and it was normal. She was checked and was negative for anemia. After another hour goes by, she seems back to normal. This does not occur after rehearsals - just the competitions. Any thoughts?