What Causes Painful Lump Under The Armpit?

hi i have a lump under my armpit that i noticed a couple months ago until now ive had no problems with it now i am starting to feel a bit of pressure pain like when you touch it a little pain is there but not bad pain just annoying pain, i m a little worried at first i thought it was a cyst and thought it would go away but it hasn t what could it be?