I have sweat induced reactions when I come in contact with nickel. Like I can wear a stainless steel watch, and as long as I don't sweat, I'm fine. If I do sweat, I end up with a rash that looks like your pictures. Sweat can be pretty reactive. Obviously this doesn't look like a reaction to metal, and I'd be surprised if it was pads, since there aren't many machines that have pads in that location. You could consider changing your detergent/fabric softener. Sometimes people can become allergic to a particular type, especially if it's scented. This could explain why you'd have this kind of reaction in the t-shirt area.