OK, I can tell you what to do to treat this if it is the typical "under the boobs" heat/yeast rash, but it is Rx, so you have to get the doc to order it. I got this from a dermatologist.
Desonide lotion 0.05% (a cort)
Spectazole oint 1% (an antifungal)
Zeasorb powder, the plain one, not the antifungal
First you apply the lotion, just a thin layer, and rub it in as best you can, it won't completely rub in. On top of that, and I know this sounds weird and wrong, but then apply the ointment in a thin layer. Of course you don't want to walk around all day with this slimy mush under your boobs, so then you apply Zeasorb powder on top. Again, I know it sounds uncomfortable and gross, but once you try it, it really isn't so bad.
Zeasorb powder is wonderful stuff.This is OTC and you may have to ask the pharmacist for it, it sometimes is kept behind the counter for some reason. About $4 for typical sized bottle of powder, about the same size as a gold bond container you'd find in pharm (ie, not the super-sized)
Once the rash clears, and trust me, it will, then you just sprinkle a little zeasorb under there everyday. I haven't had a rash ever since I have been using my daily sprinkle, and it is hotter than hades in NJ right now, I sweat like a pig in home w/o a/c in home health, and I have been in good shape. I would buy stock in this product if I could!! There is no substitue for this powder, cornstrach isn't the same, gold bond has that irritating menthol burn, I'm telling you, trust me, this stuff is different than anything else you have tried.
Let me know if it works.