Girlfriend is constantly fainting?

Hideki Asked: Girlfriend is constantly fainting?

My girlfriend has been fainting a lot in the past 2 years. She faints once or twice every 2 months and its stressful for me because I never know when she'll get a concussion or something really serious. She tells me the second she wakes up,
"My head's spinning. Why's it spinning?"
She's been to the doctor's several times(3 different doctors and they all say the same thing) and all they say is,
"It's normal for you to faint like that." Or something of the sort because of some bodily system or something.

She's not pregnant, this is a recurring problem, she's quite healthy, shes 18 years old, 5'4", 105 lbs., she's happy, she's never gone through serious depressions, she eats properly. I'm just getting worried because if I'm not there when it happens (like today), she can hurt herself really bad. I tell her if she feels dizzy to sit down in a ball so that if she faints it won't hit so hard.

I've been with her for 4 years now.