Why does my phlem smell like Chlorine?

I have had a sinus infection for the past week so I’ve been coughing up Phlegm and getting all Snotty and shit so whenever I get home when I start coughing up Phlegm and Mucus I spit it out into a little empty water bottle that sits next to me I’ve been doing this since Sunday and today I spat up some phlegm into the bottle and when I did that I caught a whiff of the inside of the bottle and it smells like a Swimming Pool and I have no idea why it does.