The lightheadedness occurs when you stand because gravity causes the blood to pool in your feet and legs. Normally, this signals the heart to pump the blood faster to circulate it to the upper regions but, for whatever reason, this does not happen. It can be caused by any number of things including dehydration, medications, heart problems, diabetes and nervous system disorders.
Since you mentioned that you work out a lot, I would suggest that you first make sure you are getting enough fluids.