The breast tissue, in addition to the obvious tissue that sits on the front of the chest wall, also extends up in a small tail into the armpit. Therefore, anything that causes tenderness or stretching of the breast tissue can be felt as pain in the armpit.
Therefore, the most likely cause of armpit pain after child birth would be something related to breast tenderness and stretching. This is almost always breast engorgement, or distention of the breast tissue with milk during breast feeding. Making certain to nurse frequently and fully empty the breasts with each feeding can help relieve the pressure. Also, wearing a good supportive nursing bra can keep the breast tissue from sagging downward and causing a stretching sensation in the armpit. Finally, making sure to support the nursing baby well so that they do not pull down on the breast while nursing will help.
If there is redness overlying the breast, fever
, or severe pain, then this may be a sign of mastitis, which is an infection of the breast tissue that is very common after child birth. This infection generally requires antibiotics, which can be prescribed to you by your primary care doctor
or your OB GYN doctor.