The lump in the armpit after pregnancy may be caused by the following reasons: 1. lymph nodes. If the lump in the armpit can slide, you need to consider whether the lump is caused by lymph nodes. If the lymph nodes are small, you can observe and treat them, but if you feel pain, you need to go to the hospital for further treatment. 2. sebaceous cysts, if the lump under the armpit does not slide and black spots appear, you can consider whether it is caused by sebaceous cysts, which can be treated by surgical removal, and pay attention not to press repeatedly, otherwise it will easily become inflamed. 3. fibroids, if the patient’s lump does not move and there are no black spots, and the lump is not inflamed, you can consider whether it is caused by sebaceous cysts. If the patient’s lump is not movable and there is no black spot, and the lump is hard, you need to consider whether it is caused by fibroids, which can be surgically removed.