This happens to me all the time because I have low blood pressure but it's got much worse with pregnancy.
In pregnancy it's often caused by anemia like everyone else has said or because you haven't eaten in a while and your blood sugar is low. It's amazing how quickly it can drop sometimes in pregnancy.
It's scary if you're not used to it but don't worry, it's quite normal. I've mastered the art of completely blacking out but remaining standing now. How cool is that!!
