Meg28

When extremely turned on, a lot of blood goes down to your nipples and your vagina...so this of course means there is less blood in your head. That's why you get a little dizzy. Some people actually pass out close to or at orgasm.
Another reason may be that you're breathing has quickened or at times you stop breathing, and that can lead to light headedness as well.
It's normal, just lie down after the act and recuperate!