How to avoid passing out in yoga class?

I%26#039;ve almost passed out in the past 3 yoga classes I%26#039;ve gone too- my vision gets really dark, and my hearing starts to fade- like a really bad head rush! It%26#039;s embarrassing to revert to the child%26#039;s pose when everyone else is doing what they%26#039;re supposed to. How can I avoid this? My instructor said that it may have to do with low blood pressure-- how could I change that? I am breathing normally too- so I don%26#039;t think that%26#039;s it.|||Stay away from yoga class or else eat better foods that provide energy and drink plenty of water before class!