I don't sweat that much and not overweight, maybe a couple of pounds, and mine are not on those areas, but on legs and arms. It just look like spots of unpigmented skin, it does not look like a fungal infection, but i'm not sure what one looks like, the only ones i've seen on tv, look like white powder on the skin in sweaty areas, in normally, overweight people it seems
I don't use soap either or anything drying to the skin.
As i've been getting slight rosecea recently as well, another autoimmune issue, and have hypo, i think it is more likely vitiligo.