you say it's not a rash, but what you are describing sounds exactly like bra rash. Just like any other tight or confining clothing, when you mix friction and sweat, you can get rashes. Bra rash is temporary and can be alleviated by wearing a different bra and of course a bra that fits, but if you have large breasts, the problem may be persistent. Try a little baby powder, a better fitting bra, or avoiding doing a lot of sweating in your bras or being in the heat and see if the condition improves.
Skin can darken when I rubs together, like dark armpits or inner thighs. I have the same thing, it's not an isue
Are you diabetic in any way? If so, I may know what it is. But I've heard of other similar cases, and people with it, use a shampoo called selsun blue, to clear it up.
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that's your brain coming out. Stop showering
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