Where do lumps from cancer normally occur?

My mother has had a masectomy 2 months ago , and still undergoing cancer treatments. I dont want to scare her just yet, but I have a huge lump,possiable a little lump connected (hard to tell) under neath,below my nipple and up under my .I know I need to go to the Dr. but I want to get through hers first.I hear most womens cancer or lump is form on their side of their , is that true ?