What Causes Lump Like Cyst In Lower Right Armpit?

I have a lump or type of cyst in my lower right armpit. My mother (also an MD) drained it until only blood remained and then put bacitracin on it after disinfecting it with hydrogen peroxide and an alcohol swab. The needle she used to drain it was a sterile syringe needle. The lump grew during my menstrual period at the end of May, but it originally occurred after an instance of shaving my armpits at the end of April, 1 day prior to having the menstrual cycle. The lump now feels like it has 2 levels. A slightly firm lower level and a malleable loose upper level. I sterilized a needle yesterday and then heated it in a flame before puncturing the upper level which seemed very full of liquid. I drained an amount of blood and liquid (not pus) and cleaned it afterward with hydrogen peroxide and applied neosporin (no bacitracin on hand.) The upper level of the lump is deflated with the hole that I made with the needle. Yes, I had a Tdap shot in December 2013, so I m covered for tetanus. But the lower level of the lump is slowly diminishing....even though it begins to feel harder to the touch. I ve thought it is an ingrown hair that is in an inconvenient place, but I require advice. So, please advise.