Hey there,
That symptom is quite common and can mean a few things. Usually its called simply "orthostatic hypotension" *sp?* and is simply a quick drop in the blood pressure when changing positions or standing. Is that the only time its occuring? If it becomes really bad or concerns you, you may want to just get some blood work done. Sometimes it can be a sign of anemia or hypoglycemia (I learned that one the hard way falling butterball naked in the shower and cracking my head on the way down).