What Causes A Lump On The Side Of The Breasts In Armpit Area?

I have a large grape-sized lump under my left armpit/side of breats area. I have felt it twice and both times it has disappeared suddenly.....within minutes all of a sudden its gone! After the first time it happened I thought it was gone for good until about a week later it returned! It is very sensitive when it is there. As soon as it disappears the sensitivity goes away! Is this just an enlarged lymph node due to an infection? I am scheduled for a mammogram in a bout 2 weeks. I am 51 yrs old.