Now she'll like us more...
There was a singer named Greg Page from a band called the Wiggles who had the same symptoms as you. He has dizzy spells all the time and actually had to quit the band because of it.
You could be suffering from: Dysautonomia
Postural Orthostatic Tachycardia Syndrome
Often more simply referred to as postural tachycardia syndrome, or POTS, this disorder is characterized by the body's inability to make the necessary adjustments to counteract gravity when standing up.
The defining symptom of POTS is an excessive heart rate increment upon standing. However, as you will discover, there are a multitude of other symptoms that often accompany this syndrome. As such, POTS can be a difficult disorder to detect and understand.
http://www.dinet.org/
http://en.wikipedia.org/wiki/Dysautonomia
Hope this helps and doesn't scare the hell out of you now!