My Granddaughter Has Had Issues With Fainting/passing Out During Sports

My granddaughter has had issues with fainting/passing out during sports conditioning and times of stress. EKG's have been done and a holter monitor has been worn. Most recently she had an intenst running workout for track followed by stretches then another session of running. During the last session she had chest pains. felt dizzy and then blacked out. She is scheduled for another EKG next week. Will the EKG show anything since she seems to have her issues during physical activity or shortly thereafter.