The crepe nest generally refers to the armpit, there is a hard lump underneath the possible causes are more, the more common are folliculitis, sebaceous cysts, lymph node inflammation, fibroma, lymph node tumors, etc., should be timely consultation to find out the cause of the disease. 1. Folliculitis: the cause of the disease is related to bacterial infection, which can lead to the appearance of pimples centered on hair follicles, manifested as hard bumps in the armpits, which can also be accompanied by redness and pain. 2. Sebaceous cysts: Sebaceous cysts are usually caused by the blockage of the sebaceous glands of the hair follicles and the accumulation of sebum in the local area, which can form cysts of medium hardness, manifested as hard bumps in the armpits, which can be combined with infections, resulting in redness, swelling and pain. 3. Lymphadenitis: the lymph nodes in armpit are rich, if there is bacterial infection, it may cause lymphadenitis, which will lead to local hard lumps with swelling and pain. 4. Fibroma: Fibroma belongs to benign lesions, usually located in the subcutaneous, which will lead to the appearance of local round or ovoid lumps, manifested as hard bumps in the armpits, usually not accompanied by other uncomfortable symptoms. 5. Lymph node tumors: malignant tumors can invade axillary lymph nodes through lymphatic metastasis, resulting in painless enlargement of these lymph nodes and hard texture. If you find a hard lump under the armpit, it is recommended that the patient consult a doctor in time and follow the doctor’s instructions after a clear diagnosis.