The main reasons for having bumps in the armpit after pregnancy include the following: firstly, it is considered to be caused by the enlargement of axillary lymph nodes, there are axillary lymph nodes in the armpit, after pregnancy, with the change of the body’s endocrine level, the local immune system function abnormality will cause the axillary lymph nodes reactive hyperplasia and enlargement, which will lead to bumps in the armpit; secondly, it is considered to be caused by the axillary lobe After pregnancy, as the level of estrogen and progesterone changes, it will stimulate the axillary lobe tissue to grow and swell accordingly, thus forming a little painful bumps. The third aspect is that it is caused by a large number of fatty tissues in the armpit.