If it's causing you to pass out literally, you need to see a doctor ASAP - you could crash a car or fall and hurt yourself. Even if you aren't truly passing out you should be seen by a health professional, too many possibilities for an Internet forum to guess about. Good luck!
It could be any number of things...you could have high blood pressure, you could have diabetes, you could also be having ear problems. That sometimes could cause the dizzyness.....also could you be pregnant?