If this happens when you stand it could be hypotension which is blood pressure that is abnormally low. In some people, blood pressure drops when they rise from a horizontal position. In mild cases, it causes light-headedness.
Severe hypotension leads to inadequate blood circulation and shock. There are several conditions that cause this such being dehydrated, blood loss from a hemorrhage, a heart defect, or Addison's disease.
Another condition that causes the same symptoms, feeling dizzy and light-headed has nothing to do with blood pressure. That is an inner ear infection or an allergy that causes an ear infection.