Depending on where it is it could be a fungus from your bra or a fungus that is contagious (I don't remember what it is called my boyfriend had it on his chest and I have it on my neck.) either way a prescription anti fungal cream should do the trick.
It is possibly a fungus of some kind.
Fungal infections are hard to get rid of, so you need to see a doctor.
Wishing you the best