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Maybe you are just "unfit", and your heart isn't able to efficiently pump enough blood into your working muscles AND your vital organs, ie brain.. It isn't unusual for people to feel dizzy and even nauseous when they up the intensity to their workout/ start working out too hard too soon...
p90x is pretty intense for the average joe as it isn't your typical body building do 5 sets of ONE exercise with 5 min breaks.
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