My 84 Year Old Mother Is Just Getting Over A

My 84 year old mother is just getting over a virus. She said her right breast was tender...When I looked at it she has a knot over it that wasn't there a few days ago about the size of a fist. She had radiation in the breast about 20 years ago for breast cancer. I do have an appointment scheduled for her but I'm still worried. She's not in pain but says it's quite annoying