If you have a hard lump inside your armpit, you can go to the general surgery department or breast department of the hospital to check it out. There are many reasons for a hard lump in the armpit, such as enlarged lymph nodes, lipoma, cysts, boils, parastomia, and so on. 1. Enlargement of lymph nodes: e.g. lymphadenitis, enlargement of lymph nodes caused by diseases of neighboring organs. 2. Lipoma: Abnormal proliferation and differentiation of fat tissue in the armpits, which may also cause a small hard lump in the armpit. 3. Cysts: Diseases such as epidermal cysts and sebaceous cysts can also cause a small hard lump in the armpit. 4. Boils: Purulent infections of the hair follicles and the surrounding soft tissues can cause hard nodules under the skin at an early stage, which can also lead to a small hard lump in the armpit. 5. Paramastia: It is a breast tissue that occurs outside the breast, usually asymptomatic, but can also appear in the armpit. There may be other reasons for having a hard lump inside the armpit, so it is recommended to go to the general surgery department or breast department of a regular hospital for further examination and treatment.