Have you been using a Sit-N-Spin? That will make you dizzy.
But seriously, folks, I had dizzy spells when I changed offices just a couple months ago. I had never had them before and it kinda freaked me out. My new office had florescent lights in it (doesn't anymore, thank god!) and I've heard of florescent light sucking the vitamins out of you. So I think I would agree with others here that it may be a vitamin deficiency of some kind.
Another thing, florescent lights vibrate at a different rate than a computer screen does, so I though it might be that, too. Too many conflicting waves. As soon as we got floor lamps in the office, my dizzy spells went away. Go figure.