My Elderly Mother Has A Lump On Her Bicep That

My elderly mother has a lump on her bicep that has come up quickly. Yesterday it was sore and the bruising looked like it was coming out. Today she said it is darker and not as sore ...it take this as the bruising coming out. She has not fallen and has no idea how she got it. Any ideas. She is adverse to seeing a doctor as she is blind and its difficult for her.