A rash under your breast or breasts, between the folds of skin is usually caused by a skin condition called intertrigo. It is a very common condition that can occur.
There are many possible causes for a rash under the breast, including infections, allergies, autoimmune disorders, and cancer. Most will resolve on their own.
A rash that develops under the breast, or on the chest, may result from skin irritation or an allergic reaction. Other causes can include infections and skin conditions, such as eczema and psoriasis. Other names for heat rash include miliaria, sweat rash, and prickly heat.