What Causes Painful Bump In Armpit?

Hi,
I developed a slight bump in my left armpit and the surrounding area near to the bump is swollen, not hard, not painful. I got this problem when I was working in a very stuffy and warm room one day about say, half a year ago. The swelling just won't go away, neither is the slight bump near to it. When I press the bump, it'll redden, but there's no pain. I did not have it before i started working in the warm room till I was overheated. Can you please tell me what it is and what's the cause? also, if this is dangerous? Thank you in advance.
Kind regards,
Chin.