Swelling/lump in my armpit

I'm a 24 year old women that's about 30 pounds overweight and has had chronic sinus infections for the last 3 years.  

Yesterday I noticed I had a bad ingrown hair in my right armpit.  Upon closer examination, I realized that my right armpit looked significantly different from my left armpit. The left side looks normal-concave.  My right armpit looks almost the opposite- there's a swelling that involves most of the area.  When my arms are down at my sides, I can still still a noticeable difference-there's a bulge under my right armpit which isn't seen under the left.  When I press and feel the area, the whole armpit is tender and I can feel a lump that is marble sized and firm under the skin-not right by the surface.  I don't feel any of this on the left side.  Also, the ingrown hair cleaned out easily and no longer causes any discomfort-it was above the area that is swollen and at least 1 & 1/2 inches away from the lump.  

As I've thought about this, I realize that I've had swelling in that armpit for quite a while-maybe longer than a year.  I never thought anything significant about it for some reason, but I'm worried now.  I had a breast examination last year and the doctor didn't notice anything.

Should I be worried?? What could this be? Do I need to get in with my doctor quick?