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.
Heat rash (miliaria) occurs when your sweat glands become blocked and sweat can't be excreted through the pores. Instead, the sweat pools under your skin.
A rash under the breast often results from skin irritation, an allergic reaction, or heat rash. Other causes can include viral and bacterial infections.
Itchy, red, sore skin rashes are never fun, especially when they're around your breasts. There are many explanations for what could be going.