Hi there, Thanks for your query. From the information provided, the possible causes are :- * Dehydration following strenuous physical exercises, due to loss of water + salts in the form of sweating and in breathing. Replenishment of fluids alone is inadequate. A pinch of salt in each glass of water should be taken liberally before and after exercises. * The blackout too was apparently due to either dehydration; or, a precursor of migraine called ocular migraine . I would recommend that your daughter keeps her well-hydrated along with table salt. She should improve and not have either dizzy spells/light headedness or blackouts. If she continues to have momentary blackout followed by a prolonged headache, a diagnosis of ocular migraine should be considered. In that event, consult her doctor who will put her on long-term preventive treatment for migraines. Hope you find my reply helpful, informative and reassuring. Dr. Rakesh Karanwal