Decreased blood. To the brain. Usually caused by low blood pressure or? Mild stroke, block in neck arteries, or heart problem. Can be normal in young. Need evaluation as soon as possible.
Decreased oxygen. The eye, like the brain, has a large amount of blood flow relative to its size. Any small decrease in blood flow will cause the retina (film layer) to have a slight temporary decrease in oxygen, making it "scream" for more. Lightheadedness can be due to low blood flow to the head. So it is not uncommon for both of these phenomena to occur together. See your doctor if this condition persists.