Doesn't sound like fungus or yeast to me, especially if treatment for that didn't work.
Much more likely is irritation, from skin rubbing against skin. I suggest you try over-the-counter 1% hydrocortisone twice a day. If that fails, see a doc for a somewhat stronger preparation.
This condition may recir, but it isn't catchy. And by the way, it happens quite often in folks who aren't overweight.
Best.
Dr. Rockoff