Go to the Doc... I had that same feeling when I was young. (starting in 6th grade) I would actually pass out running in gym class.
Turns out I have a mitral valve prolapse... it is pretty common (more in women than men)
here's a web site
[url="http://216.185.112.5/presenter.jhtml?identifier=4717"]http://216.185.112.5/presenter.jhtml?identifier=4717[/url]
I got an ultra-sound done years ago to diagnose it! Good luck - Cathy
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God grant me the Serenity to accept the things I cannot change; Courage to change the things I can; and Wisdom to know the difference.