You could be allergic to the elastic or to the fabric. Either that or they are an irritant. Because the rash is limited to that area it is doubtful that it could be anything else.
Actually I take that back. It could be prickly heat. That would be a yeast infection. Desitin would clear it up.
It could be simple yeast infection or a reaction to Victoria Secret's bras. People are reporting severe reactions to the formaldehyde.
And it could be something like athlete's foot, a fungal infection but not yeast, given you say it occurs where you sweat. Yeast generally needs to be inside the body to survive, but there are many other fungi that cause rashes. Raw garlic, oregano oil, neem oil are just three anti-fungals that might work. But they won't if it's an allergic reaction. And they need to be taken internally and externally often to work, and natural anti-fungals don't smell too good. But they also don't kill off beneficial flora that prevents such infections in the first place, which pharmaceutical products do. Did this start, by the way, soon after taking an antibiotic?
I don't wear Victoria's secret. Is there formaldehyde in other brands? Because I am sensitive to all kinds of chemicals since 1982. I had carbon monoxide poisoning slowly and it killed off my immune system and I had to take pharmaceutical grade vitamins to rebuild it, but it still left me with allergies to just about everything but food.
haven't taken antibiotics for years, in fact, haven't been to a Dr's office in 8 years. Got tired of them finding things wrong that didn't seem to be affecting me and giving me drugs. When I quit taking their drugs, I didn't feel any different than I did with them.