Hard lumps under the armpits after pregnancy can be caused by factors such as breasts and swollen lymph nodes in the armpits. 1. Paramilitaries: The estrogen level in the body will be elevated after pregnancy, which can cause the hyperplasia and swelling of the paramilitary glands in the armpit, resulting in the appearance of hard lumps under the armpit, which can be accompanied by redness, swelling, pain and other uncomfortable symptoms. 2. Swelling of lymph nodes in the armpit: the rise of estrogen and progesterone in the female body after pregnancy can cause the expansion of milk ducts and hyperplasia of the mammary glands, which can lead to swelling of lymph nodes in the armpit, resulting in the appearance of hard lumps under the armpit. 3. Others: Sebaceous cysts and fibromas in the armpits may occur after pregnancy, which may not be obviously related to pregnancy, and may also appear as hard lumps in the armpits. If you have hard lumps in your armpits after pregnancy, you should consult a doctor in time and take appropriate treatments under the guidance of a specialist, which will help to alleviate the uncomfortable symptoms as soon as possible.