Gasping for Breath After a 25 yds

I'm 64, slightly asthmatic, in decent shape from years of bicycling, and swim in a pool disinfected with chlorine and ozone. I have a decent freestyle, but can't swim more than 25 yds. without gasping for breath. My three theories are: 1) my heart muscle just doesn't have the strength to send the blood to my muscles, 2) my wind passages are closing up as an allergic reaction to the ozone, and 3) my stroke technique prevents me from getting a good breath of air. As to No. 2, if you google "ozone allergies", you'll see articles talking about how ozone triggers an asthmatic attack in asthma sufferers. Does anyone have any theories about why I can't swim more than 25 yds.?