Painful red bumps in one armpit? varying sizes some look almost like pimples, what is this?

Hair follicle infect: Most likely you have infected hair follicles. If you shave under the arms, this can occur and even if you don't. If there are enlarged lymph nodes under your arm, you may need to be seen and given an antibiotic. Sometimes these develop to the point of having to be opened up. Soap and water, avoiding deodorants if one can and very good hygiene are important. If fever or streaks, get seen.