A red bump under the armpit and itchy. There are several common causes: a. Allergic reactions, i.e. skin allergies. After allergy, a rash forms around the skin, with particles of varying sizes, protruding from the skin and reddening. Anti-allergy treatment can be carried out, local anti-itch treatment. Second, peripheral tissue eczema. Eczema can also occur as large red bumps with more pronounced itching. Treatment is also based on anti-allergy treatment, supplemented by maintaining local hygiene, but the medication needs to be used under the guidance of a doctor, do not treat blindly.