How To Get Rid Of Black Bump Under Armpit?

hi i am asking you dr andrew i got this black bump under my left armpite that i thought it was a pimpel i tryed poping it and it hurt really bad it was a littel red and tinder but when down that was a couple months ago i noticed today that it is still there and have not heald up look like it is comeing back as a pimpel and it is black what should i do thanks harold