Young, active female (27), fainted for the 1st time during a moderately strenuous workout. 2 days later, still feeling light headed on occasion.

Fainting: or "syncope" as we refer to it has many possible causes: neurologic, vascular (like low blood pressure), heart problems, electrolyte imbalance, anemia, etc You should make an appointment to be checked by a physician, have lab work and other studies if deemed necessary, especially to rule out any serious conditions. Even young, healthy, fit individuals can have unidentified medical problems.