Hi,

Could either be your lymph nodes if your a bit sick at the time or milk ducts/glands.

My immediate thought on reading the title of this thread was milk ducts. My SIL, when her milk came in discovered she had milk ducts all the way into her armpit and down to her elbows. When her milk came in she got ENOURMOUS lumps under there and could actually express milk from her armpits. But thats another story lol. Quite uncommon to have it to that extent but yeah.

So it could be that your milk is starting to come in