What Causes Syncope Without Exertion?

My 16 year old niece was in a crowded place with friends, and she began to feel faint. They took her to the nurse and her blood pressure was 135/120. After sitting in the cool for 15 minutes her pressure returned to normal levels... 121/75. Could heat have been a factor here? what else would make her pressure rise and return to normal so quickly without physical exertion?