Can somebody tell me what this is that my gf has?

my gf has this rash on her, dr doesn't know what it is, but i been telling her he's a quack and doesn't care anyways. but my brother has it as well and he gets it when ever he works and sweats, but she doesn't listen, and both me and my mom have showed her but she still doesn't listen. but she went online today and found something call PUPP, idk what it stands for. but its something for a pregnant woman, but she's not pregnant we've done gone to the dr (different dr) and he said she wasn't pregnant, had a ultrasound and some other things done and that was like 2 wks ago. and its been on her for quite a while now. could it be the same stuff but from when she was pregnant with our daughter? she's 22mo old today. looks like little red circles under her boobs, like she has to lift them up a little bit so that you can see them. could it be from the bra's she wears?