I am not a doctor, and I'm glad you had it medically checked out. My guess is it's just your system all revved-up, more than is necessary for the situation, extreme fight or flight mode. When I get really nervous or afraid, my heart pounds, and this is happening less and less as I get more relaxed and see more clearly. Just a simple tapping to the side of your collarbone might slow you down sooner, EFT-like. Keep an eye on it -- maybe you are pushing yourself a little too hard.
2012: 20/45 on average, no glasses except for night driving
2001: 2/200, -10 hard contacts with -1.75 cylinder
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