It's not unusual for people to get bruises and not remember the cause.  If this is really a bruise, it will start improving in a few days and be gone completely in a week or two.  If that happens, she doesn't need to worry about this being any kind of cancer.  If she frequently has bruises and doesn't remember how they happened, she needs to see her doctor because bruising easily can be related to medications or underlying medical problems.

If the bruise doesn't go away, then your mother will need to see the doctor to find out what it is.  This article on some rashes common in the breast may be helpful.  Discoloration of the breast is not a typical breast cancer symptom, but one rare type of breast cancer--inflammatory breast cancer--can look like a bruise.  Usually IBC also would include swelling, skin dimpling, and other symptoms, so please don't panic about this being cancer.  Do take action if it lasts longer than a week or two.