What could cause a headrush feeling or a falling feeling while sitting? only for a few seconds

Near syncope: Or alterations of brain perfusion from positional motion... Can be positional vertigo, could be arterial dissection. If you faint or hit your head your must go to the emergency room and or see your doctor for this. The main concern is what if this happens when you are driving or walking and might hit your head. Sometimes a lesion in your carotid body can change your brain perfusion.