How to avoid passing out in yoga class?

I’ve almost passed out in the past 3 yoga classes I’ve gone too- my vision gets really dark, and my hearing starts to fade- like a really bad head rush! It’s embarrassing to revert to the child’s pose when everyone else is doing what they’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’t think that’s it.