it would not be possible for me to diagnose your daughter over the internet based solely on the information in your brief posting. there can be dozens of reasons but it is going to take a detailed medical history a careful examination and perhaps a few diagnostic tests (blood tests) in order to narrow down the many possibilities. this is your first step.
my daughter 15 works out 4 days a week takes vitamins but seems to gte sick every few weeks is she just run down? . she eats fruits and vegetables. could eat better but wanted to make sure i wasn't missing anything.