Just because you are drinking "diet" soda doesnt mean that your body isn't tricked into thinking you just ingested a shitload of real sugar. By the time it realizes that the crazy ass sweetness was just a fucked up chemical that can't be negotiated, your insulin levels have already skyrocketed.
Add a shitload of caffeine and some overly sweet wierd ass chemicals, hell the next thing you'll be talking about is diabetes.
Blood sugar and caffeine rushes have an affinity for causing dizziness...that and all the other dope and stress you are dealing with.