Why is my armpit so sore?

Before my menstrual period it isn't uncommon for me to get an extremely sore lump under my armpit. I've gotten one there for years. Pretty much since I first started getting a period. I was just wondering what this could be..? Because it gets SO sore. I barely even have to touch it to tell how sore/tender it is. This time there is no lump but my armpit feels exactly how it usually does and is most intense where the lump usually appears. Thanks in advance.