I used to faint... and it seemed to be at random... but universally it was caused by a couple things...
If I stood in one place and locked my knees for too long... out I would go. It happened during choir performances and sometimes doing dishes LOL!
The other main culprit was not eating for a long period. I was a child who would just forget to eat! I would suddenly run out of fuel and all of a sudden I would turn pale and my eyes would "go black" and i would wake up on the floor. If I ate a healthy snack with a little bit of carbs (like an apple or a granola bar) I would be as good as new!
The doctor eventually said that it was because I was low weight (natural bean-pole) and I had mild anemia... Iron and remembering to eat (and shift my weight when standing still) prevented many more episodes.
If you are worried, certainly get her checked out, but a few fainting spells are not necessarily a reason to panic. Do take her to the pediatrician and ask if they can test her blood for any deficiencies. Chances are this can easily be treated and prevented with simple measures!
Good Luck!
-M.
PS... I grew up to be a normal, pretty healthy adult!