Intramammary lymph node is a common benign finding in the breast, typically seen in the upper outer quadrant. However, further workup may be indicated if there are atypical features, such as absent fatty hilum, thickened cortex, not circumscribed margins, and/or increased size or interval change, the latter which may be applicable in your case. In all likelihood, the intramammary lymph node was probably present on the prior studies. They are so common that they sometimes not mentioned. Before going overboard with the MRI, it might be a good idea to request for the radiologist to review the prior studies to see if the lymph node was present in retrospect and make an addendum if necessary.
I assume you have talked it all over with your doctor; are you here in order to see if what your doctor has told you is correct? (If you haven't discussed your questions with your doctor, I'd certainly do that before asking on the Internet.)
Regarding how worried you are, how old was your aunt when she got breast cancer? My doc once told me that if an aunt got breast cancer post menopause, that is much less concerning in terms of family history. He said that later in life the body has been subject to so many environmental insults that the likelihood of cancer is increased overall.
Second, a "node" seen in a mammogram is usually a lymph node, I think. (But of course, you should check this with your doctor.) Apparently lymph nodes can show up on mammograms and it doesn't mean much. I've also read that if a lymph node is visualized on a mammogram due to cancer, the cancer would also be clearly visible.
Anyway, your doctor, not Dr. Internet, should be your guide on what further tests are called for, if any.