Lump And Swollen Lymph Node In Armpit. Recurring After Taking Antibiotics. Done Mammogram. Could It Be Cancer?

I discovered a pea sized lump and swollen lymph node in my right armpit in April 2009. I went to the doctor and he also discovered a small rubbery lump on the side of my breast very near/borderline armpit. He gave me antibiotics for three days and sent me for a mammogram and ultrasound . When I returned the rubbery lump had diminished but the lymph node remained and went down after a few weeks. He told me that my breast were dense/50-75% fibroglandular and I should have the mammograms and ultrasounds together every 6 monts to see if there were any changes. I had another in Oct 2009, March 2001, and the most recent was Sept 12, 2012 and on Sept 29, 2012, the same thing happen like in 2009 and the same side. Should I be concerned about something more serious? By the way I must add that I am in a Birads 3 category - probably benign. Could they have overlooked something and if it was cancer would the lymph node go down and return 3 years later or would it have gotten worst?