Lymph node vx. infxn
There are lymph nodes in the armpits which can be felt as little lumps - sometimes we can feel them even if there is nothing wrong.
Also possible to get ingrown hair
, infected follicle
or boil in the armpit if red/tender. If small, moves when you touch it, and no pain, it's fine to watch & wait, but if red, sore/painful or increasing in size, get checked by a doctor.