What Causes Intermittent Fainting Spells?

My 21 year old daughter has occasional (once a year) fainting spells. They ve been caused by excess cold, excess heat, dehydration, and nothing. Her BP is low (familial trait) and she had a spell of anorexia 9 years ago. She has difficult menses. She is a vegetarian, but cooks and is careful about eating healthfully and enough. She drinks alcohol only rarely and doesn t do recreational drugs. She s had heart and neurological workups - and we thought she d outgrown this. Today she fainted in the shower. Why? People don t just faint, do they?