What would cause postural hypotension triggered by heat in healthy 15 yr old girl. not dehydrated, good diet, slim, exercise reg. deficiency? vitamin?

Orthostatic: Postural hypotension occurs when something interrupts the body's natural process of counteracting low blood pressure. Occasional dizziness or lightheadedness may be relatively minor — mild dehydration, low blood sugar, or too much time in the sun or a hot tub. It may also happen when you stand after sitting for a long time, (class, concert, church). If only occasional, not cause for concern.