Underarm stains in shirts?

Underarm stains in shirts?

Does anyone know of anything that can cause perspiration stains in clothing to change color? I washed a bunch of t-shirts that previously didn't have noticeable underarm stains, and when they came out of the dryer, all of the armpits are pink! Is there any way to get this staining out? Most of these shirts are pretty old, so I'd rather not use bleach as I'm not sure if they can take the fiber damage that it might cause. Thanks for any help...
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