Sorry, gindie, about your mom's bad news. Hopefully she'll be one of many for whom good, effective treatment is available. Still, it's pretty scarey and upsetting, I know.
Mammograms may be about the best we have in general use, but they are far from perfect. Not only do they miss some cancers, but they identify far to many benign lesions which leads to surgery, anxiety and occasional complications. MRI is more sensitive but unveils even more false positive wild goose chases.
Best wishes for your mom's recovery.