I found a small pea-size lump in the middle of my left armpit a few days ago and was really worried. but after a lot of research ive found that its most probably from antiperspirant and spraying too close to the skin, which i do a lot

i only noticed the lump when i was washing, so it hadn't been there long and was not causing me pain, just slight discomfort when i pressed down.
4 days later and it's gone down a lot, also my friend said she gets a similar lump along with PMS, which seems just as likely that thats what my problem is.
I'd suggest to leave any lumps for at least 5 days or until your period has been and gone so that menstrual cycle causes can be excluded. dont prod and push down on it all the time as this could make you paranoid that it's more painful, when its probably from all the prodding! obviously, if you are having more severe pain than mild, go and see a doctor, or if its very hard or large/swollen.