The Monsignor of my elementary school would say that it was the result of committing the sin of self abuse.
Interestingly, around the time I graduated from high school he was completely blind...
Anyway, it's probably due to changes in blood pressure, which can affect the shape & focus of your eyes. Still, if you can, do have your cardio checked out by a doctor. Note: it would be more socially acceptable to say that it sometimes happens when you exert yourself. However they are your doctor and can handle the full truth, too.
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“For small creatures such as we the vastness is bearable only through love.”
―Carl Sagan