There are many possible causes for the lumpy feeling in the armpits, and one of the causes is secondary breasts.

What is side milk?

Secondary breasts are mammary tissue that forms outside of the native breast. It may also have a nipple and areola, or it may have no nipple or areola, and the extent to which it possesses mammary tissue varies.

Why do we have side boobs in the first place?
Mammary tissue that develops in the mother's abdomen during the embryonic period usually regresses except for a pair of breasts, but this regression may be insufficient for all but one pair of breasts. This mammary tissue found outside the breasts is called adnexal breasts.

Locations where side milk is likely to occur

The most common areas of secondary breasts are the armpits, the lower part of the breast, and the vulva. These are located on a line known as the milk line, which runs from the armpit to the base of the leg.
For example, it is easy to imagine a dog having many nipples.

Compared to Westerners, Japanese have a higher incidence of sarcoma, with about 1 in 10 people having sarcoma. About 90% of the sarcomas are found on the sides of the breasts.

The texture of the sidearm is the same as the texture of the breast to the touch. Like normal mammary glands, they also show changes caused by female hormones, and they become large and tender before menstruation and during pregnancy and lactation, which can be painful. Parac breasts that enlarge during lactation become smaller when lactation is over.

If the appearance is noticeable and bothersome, treatment is surgery.