Possibly a folliculitis. Apply mupirocin creme if it is pointing.
Is the lump/bump only on the one armpit or both? Approx how big is it?
I think i have experienced this myself and i came to the conclusion that the lump or bump came after me shaving my arm pits and/or using too much anti-perspirant deodorant or even just holding it too close to your armpit when spraying. Your armpits are very sensitive, you may just have caused an irritation. Try and use a deodorant that is for sensitive skin. If it gets worse i might be wrong and i suggest showing a trusted adult or going to the doctor.
Hope this helps.