Hello! I have been through your question.
Related your concern I think this could be yeast (
Candidiasis) infection. Furthermore you should know that when a
rash is present in an area where skin on skin contact occurs, this is known as
intertrigo. Intertrigo is inflammation of the skinfolds that is caused by increased moisture and friction in the areas where skin meets skin.
The area with intertrigo can produce intense itching, burning sensations and it may also have a terrible foul smell similar with your symptoms.
The inflamed skin under the breasts can be an open door for Candida. What I suggest for you is to keep dry your underbust,minimize moisture and friction with absorptive powder, wear light, non-constricting,and get the right type of bra. Meanwhile meet a doctor to start local antimycotic therapy.
I hope my answer helps you.
I wish you a quick recovery.