A small hard lump inside the armpit hurts when pressed, which may be trauma, inflammation, tumor and other diseases. 1. Trauma: Due to trauma to the area under the armpit, the surrounding tissues become congested and edematous, which may lead to the appearance of a hard lump in the armpit that hurts when pressed. 2. Inflammation: axillary lymphadenitis when the lymph nodes in the armpit are infected by external pathogenic bacteria, the formation of lymphadenitis will cause lymph node enlargement; skin follicles and the surrounding tissues of suppurative infections, inflammation in the early stage will cause subcutaneous hard lumps of red, swollen and painful skin, which will lead to armpit pain when pressed with a small hard lump inside. 3. Tumor: such as lymphoma, breast cancer, etc., due to lymphatic metastasis resulting in swelling of lymph nodes in the armpit, involving the armpit tissue, resulting in the emergence of a hard lump in the armpit pain when pressed. There is a small hard lump in the armpit that hurts when pressed, and there may be other reasons, so you need to go to the hospital in time for examination and follow the doctor’s instructions for treatment.