What Causes Lump In Uderamrm Before Periods?

Hi. I am a 29 year old female and I realized that before my period i experience pain and lump under my right armpit. this has been happening for the past few years and now my right underarm is bigger than my left. my right arm is noticable bigger. can you explain what is it? I went to a doctor and he told me it has to do with my hormones.