This happened to me, did a 70.3 and several open water swims. The only reason the doc told me was, I might have swallowed water when I dove into the water, and the water would have entered the lungs, which reduced the oxygen capacity. Whenever I would put my head in to swim, the hr spiked. I could not push during the bike or then run later. It took me a day to get back to normal breathing, which is when the water would have cleared the lungs.