I often have this happen .....usually when I've been shaving with a razor that hasn't been changed in awhile. I find it helps if I change my razors more frequently and cleanse my armpits daily with an antibacterial soap.
I'm no doctor, but I'd say that a lump that stays there and grows larger is something that's more concerning than a swelling that comes and goes. If your worry persists and/or the swelling gets worse (or doesn't go away in a few days), then I'd suggest making an appointment with your doctor.
I have had a boil under my arm. It started the same way but continued to swell for about a week then popped and oozed until the core came out then healed. This was very painful. After that a few times I thought I was getting another one but it settled down and never happened again.
Hope this helps.
This happened to me 30 years ago. I discovered that I was allergic to certain deodorants. I switched to Clinque, which is hypo-allergenic. No problems since. I also do not use deo soaps, like Dial.
HOpe your cure is simple.I would see your DR ASAP, it could be lots of things.
omg i have same problem as i speak right now i saw the lump under my armpit smh lord jesus i am scare i also shave from alot cause hair underarm grow fast i shave it alot i have this feeling armpit lump could be the problem