What can be causing a red itchy rash under breasts?

Rash under breasts: Skin fold areas, where skin is pressed against skin, is prone to heat and sweat rashes. These areas include the upper inner thighs, under arms, and skin under the breasts, especially women with large breasts, you should dry the skin there by using witch hazel, several times a day, and allow the area to dry by fanning. Get Hydrocortisone cream 1% and rub a tiny bit once or twice a day.

Fungal? : You may have problem with a candida (yeast ) infection especially if your breasts are quite large and you sweat a lot. Other potential but less likely cause would be a contact allergy to material used in your bra.