Some people are prone to getting lightheaded, especially people who have fainted in the past. Doctors call simple fainting spells vasodepressor syncope.
Normally when blood pressure starts to fall our heart rate speeds up to compensate. During a simple faint, blood pressure falls and heart rate does the opposite of what it should -- it slows down. If you are standing, not enough blood gets pumped to the brain.
Have you had fainting or near fainting spells in the past?