I Have An 8 Year Old American Bull Dog- Female

i have an 8 year old American Bull dog- female with a hard golf size mass in one of her nipples. It has grown in its current size in the pass year. She has been spayed after she had a liter of puppies about 2-3yrs ago. I know there's a higher chance of being breast cancer, but I hear that its true if she was not spayed. What can you tell me about this?