MBC with sudden breast changes - fungating or mastitis?

Hi all - I’m new here, seeking advice and support for my 75 YO mother who has been living well with MBC for over 2 years, until this past Mother’s Day when they identified her GI discomfort as liver and pancreatic mets. Her breast lump (she declined all conventional treatments: surgery/chemo/rad etc.) never seemed to have grown or changed in all this time. Overnight, she has developed sharp pains in the breast now with swelling, firmness and redness. I am not sure what the best course of action is on a Saturday morning - urgent care? ER? Cold/Hot compresses etc.? I realize that the tumor may be fungating but also very hopeful that this is mastitis and can be treated….I would appreciate hearing from those of you with personal experience or knowledge about sudden breast changes in the setting of MBC.