It's called orthostatic hypotension, if you are describing what I think you are describing. Like when you get all faint like you might pass out when you stand up?

Yeah, it's called orthostatic hypotension, and basically it's when the blood drains too quckly from your brain. Standing up more slowly can be helpful. If you've been thinking hard, it's less likely to happen, because your brain has more blood in it during those activities, and therefore, it doesn't drain out to the point that you feel dizzy.

Hope that makes sense. It's very common, and can happen more often if you haven't eaten, if you've just been asleep, etc. You can google it for more information.