well that's kind of weird isn't it? i'm sorry you're going through this. the shoulder and arm area is an odd place to get a rash or a breakout of something like atopic dermatitis. if the rash is happening only on one side it's probably worth having a health care provider look at it though it's probably not any sort of emergency. if the rash is occurring on both shoulders/arms then you might consider causes such as laundry detergent (mild allergy or reaction) dryer sheets wearing new clothes before washing them a change in the type of soap you use (again possibly a mild allergic reaction) or something along those lines. if the rash/blotchiness doesn't clear up on its own or if it gets worse spreads or your begin having swelling of the face/mouth and/or shortness of breath then you need to seek medical care. best wishes as you try to solve this mystery!
i have skin redness/scratchlike rashes around my shoulder and arm area not from scratching but it looks just like it. . it does not itch. no bumps or soarness. just red scratch like rashes and blotchy arm redness.