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My sister has something like this, has to do with low blood pressure--most people, if they get active, their blood pressure goes UP, but those with this symptom have a sudden drop in their BP and it can result in fainting.

It's why Greg the yellow Wiggle had to quit being a Wiggle. :*(

Greg Page (Wikipedia)
Illness and retirement

On 30 November 2006, The Wiggles announced that Greg Page would retire from The Wiggles due to poor health.[2]

Page had experienced health difficulties since December 2005, when he underwent a double hernia operation and withdrew from his group's U.S. tour after suffering repeated fainting spells, slurred speech, fatigue, and trembling.[3][4] Although Page was missing for virtually all of the late 2006 U.S. tour, audiences were informed of Page's absence at concerts moments before the curtain went up.[5] At first, Page was told that he had seven years to live,[4], but he was diagnosed with a non-life-threatening and difficult to diagnose chronic illness called orthostatic intolerance, which causes symptoms such as fatigue and loss of balance. Specialists believed that Page had mild episodes of the illness going back twelve years, but symptoms worsened after his hernias. It was decided that Page would retire from performing with The Wiggles to better manage his health.[2] In the months following the announcement of Page's retirement, he received an "overwhelming outpouring of public support". He reported that it took his parents six months to respond to the "mountains of letters, emails and sympathy notes"[4].

Page was succeeded by Sam Moran as a full member of the group.[2]

Thanks but I don’t see much there to account for a very strong, very active teenage boy having a problem of being dizzy immediately upon standing and then having the problem go away by early adulthood. I have three brothers and I think they all had it at about the same age. I know my older brother had it and sometimes would have to grab something to hold on to. My mother said she had the problem as a young girl and it went away once she reached adulthood.
I don’t think my father ever had this problem or maybe he just wouldn’t admit it if he had.