coughing after swimming

I noticed one thing. When I go to swimming pool, I start coughing. Is it because of the chlorin in the water ? Is it because of the packed air ? Is it because of the exercise that my lungs are getting ?
I feel like its because of the packed air. But swimming pool is big enough and when I am in the pool (not is water), I feel ok. But after 30-40 minutes, I start coughing. I have little arthritis in my knees so this is the best exercise I can do and Iont want to give up on this.
This did not happen in the beginning for 3-4 times. But once it started , now coughing is with me all the time.
Please advise on this.

thanks
 
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