i think that's called postural hypertension. your brain probably just wasn't getting enough blood. your blood pressure lowers when you're at rest, and if you suddenly got up (posture change) from a laying/sitting position your blood pressure drops (which is that lightheaded feeling you get sometimes). probably not an issue, just sit down and put your head between your knees when it starts to happen (directs bloodflow to the brain) but if it happens often you might want to see a doctor.

