This happens all the time to me, it's the scariest when I'm exercising. I've had moments where I nearly passed out in the middle of the yoga studio. I've learned to move very slowly and carefully whenever I stand up out of forward folds or just sit up/stand up in general. It's hard too when you're around other people and trying to hide it because it happens so often. This was kind of a wake up call for me- I didn't realize how much this was affecting my body and now I'm trying to recover.