I would absolutely get her to someone who deals with breasts all the time -- a gynecologist who's doing breast exams all the time or as someone said below, an oncologist with a specialty in breast cancer. But you likely would have to get her primary physician to refer her to any specialist and I would wager that her primary will brush you off and say the ultrasound is the final word.
Fortunately, ob/gyns are considered "primary care doctors" under the law so she should be able to see one without a referral and much sooner than if she gets a referral-- she should still check with her insurance. Whenever I've had issues with my breasts (have fibrocystic breasts) I go straight to my gynecologist--she handles breasts all day every day and frankly, primary care doctors just don't, even though many do breast checks and gyn exams.
Inflammatory breast cancer is fast-moving and serious, and while it is MUCH likelier that your mom does indeed have an infection of a duct or something like that -- it is worth being sure.