There is a epidemic of boils under armpits and any other dark area it is actually a staff infection and it is very contagious use warm compresses to bring to a head and then drain it cover with bandage after cleaning it with very hot water and a drop of bleach, or take a bath and use 1/4 of a cup of bleach in veryhot water.
You should make an appointment with your reg. dr. This may be something minor, though, like a sebaceous cyst or something. Usually, those can be treated with warm compresses, but any time you're not sure about something like this, you should have it checked out.