Hi BillieJean
Us women are a bit prone to cysts & boils in our armpits, due to infected hair follicles from shaving. I had a huge cyst once & had to get it lanced

The relief afterwards however, was enormous!
There's also lymph nodes in our armpits which can enlarge & sometimes not reduce again. My dermatologist found a lump under my arm during a routine 3 monthly post melanoma examination. Thankfully, after having two large lymph nodes removed, they were found not to be malignant.
As Rick says, the odds are in your favour & it's very likely you'll be kicking yourself for worrying so much over something that turns out to be fine. You really need to get it checked out though because there's no way of knowing what it is until a doctor does the biopsy or removes it.
Good luck & let us know how you get on.
Lidia