One of two items.
Water accumulation from accidently breathing in water (I take it you are swimming and not just bobbing around).
Diaphragm muscle stiffening due to the cold, which is really due to energy issues and calcium efflux from the muscle. You may notice your muscles becoming more rigid and and your movements less flexible. You're expending energy trying to keep warm at the expense of energy for endurance exertion.
Could be a combo of both.




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