JLeslie is correct. The rash is caused by a yeast (fungal) infection. It’s the same thing as a vaginal yeast infection, just a different location.
There is something better than the cream. It comes in a spray can so it’s in powder form. The reason it’s more comfortable is because the powder form of anti fungal ingredients dries everything out and prevents further rubbing and chafing.
The best brand to use, in my opinion, is LOTRIMIN in a gray can. It’s OTC at any pharmacy.There are other brands but Lotrimin is the gold standard.
However, it’s marketed for either Athletes Foot or Jock Itch. Thats what the front of the can says, but don’t be put off by that. Yeast is yeast is yeast regardless of the location. Anywhere it’s dark and damp can produce an overgrowth of yeast which requires an antifungal to deal with it.
When I’ve used it, it clears everything up in a day, but I usually use it for two days just to be sure. But the relief is pretty much instantaneous.
You can use the cream form if you want but the powder dries everything out better.
(Just be aware that the first time you use the powder spray, it will sting but only for a few seconds LITERALLY; this is due to the aerosol propellant used and likely also contains alcohol, which also has a drying effect.)
To prevent re-occurence once the rash clears, just keep it as dry as possible in that area. You can use regular talcum powder or corn starch plus wearing cotton blouses, t-shirts, etc. (as well as cotton bras). At night you can fold a piece of paper towel in half under each breast to absorb moisture.