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When in my younger days, if I did an all out, full body workout, I'd see "the white buffalo". That's what the gym rats who stopped me from passing out headlong into the squat rack called it. Im not sure that it's physiologically possible to suck up too much blood into your muscles and starve your brain of oxygen, but I believe it may be caused by rapid blood pressure changes caused by intense workouts. Just a theory. Good luck.
That's usually a sign that you haven't eaten enough before the workout. Also be sure to be drinking plenty of water and that you don't get dehydrated throughout your workout. Not eating enough in your meal prior to your workout or not drinking enough water before can cause dizziness and fatigue. Hope that helps!