At a loss to figure out what's wrong with them

Whenever I buy my sons new swim shorts, they get a rash. The problem is they only get this rash in certain areas, usually around the waist (but not all the way around, just in 2-4 spots), and their thighs at the bottom of the shorts. AND they only get it in our pool, not when we go to the lake.

We've checked the pool chemicals, and check them daily, and they are at the levels they should be. They've been checked for allergies, and are not allergic to any of the chemicals in the pool. They do have a mild allergy to latex, which may be in the elastic of the shorts, but this doesn't happen when they wear other things like underwear that have latex in the elastic. It also, again, doesn't happen everywhere, just in certain spots. We've changed brands of chemicals, too.

The weirdest thing is it's only when they're new. My boys usually can wear the same shorts 2-3 years. The summer before last, they were new and we had this problem. Last year, same shorts - no problem. This year, new shorts - problem.

I've washed the new shorts repeatedly in case there was something on them causing it. I know it's not the laundry detergent because this doesn't happen with any other clothing.

I'm at a total loss here. Does anyone have any suggestions for anything I haven't thought of yet? The rash is kind of red, not raised, and doesn't itch. But it does sting and hurt, they say. But it only stings and hurts while in the pool. Please help if you can!

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