I have an underarm mass 6 years ago and it increases in size during my first pregnancy and during lactation what could be this?

Lymph node?: Ask your doctor. They might do a biopsy or other lab work. That you have had no other problem for some years may mean that it is nothing to worry about, but check it out to be sure. It is a location of major lymph nodes. Good luck.

Possible breast tsu: When the breast tisssue matures, most of it's functional tissue is located in the primary breast area.However, some can stretch back toward the axilla on either or both sides.I remember one mom whose axillary masses on both sides were so uncomfortable, she quit breast feeding due the pressure and both masses shrunk.