I have read your query & understood your concern.
The itchy rash under breast is common in fat,diabetic,pendulous large breasts.
In such conditions, there is lot of sweating,the moistness helps breeding of bacterias/ fungi ,thus a skin infections develops & itching increases.
simple sweating can produce prickly heat
rash ( Cleanliness+coolness+ talcum powder help.
Fungus infection needs to be diagnosed by skin specialists & treatment accordingly. Allergy
to material of under garments- stop use of the allergen/ use cotton garments. Bacterial rash
has pus filled tubercles-need antibiotic .
Please consult a skin specialist for diagnosis & advice.