Pimple under my armpit???

It's likely an ingrown hair. This happened to me once too except, as a guy, I don't shave my pit hair all the way down. I don't want it to get out of control so I just trim it up. In any case, this was a new hair so it's actually been YEARS (I'm 32 now so thanks for dredging up that memory for me! *lol*) since it happened. But, being curious about it like I was I decided to try and pop it and got the same result you did except I noticed that a little hair was sticking out of the abscess. I tweezed it and never had another problem out of it but I can still see the scar where it was. I shudder at the thought of how bad it might have gotten if I'd left it alone like my dad suggested.