Many things can cause this type of problem. Your are correct that sometimes it can be caused by something serious such as cancer. It can also be something minor such as a benign polyp. Fibroids don't usually cause bleeding after menopause.
Your mother's doctor has been evaluating her all along which is good, and the D & C should shed some answers. The doctor might also want to do a hysteroscopy which uses a telescope to look inside the uterus and be sure everything is OK. This can be done in the office and sometimes is done in the operating room. If you do not get an answer after the D & C and the symptoms continue, consider getting a second opinion just to be sure nothing else could be done to find a cause.
Machelle M. Seibel, MD