Okay I'm Doing This For My Girlfriend Because She Won't,

Okay I'm doing this for my girlfriend because she won't, she has a knot on her left breast right underneath her armpit. It's causing consistent pain, but more when she touches it. Since it's been there she's suffered a loss of appetite and has rapidly lost weight. Almost fifteen pounds in three weeks. Do you know if it's something serious, we should be worried about?