Lump in armpit

So, I was putting deodorant on yesterday and it felt strange. I felt my underarm and there is a lump that was about the size of a pencil eraser. Just a day later, today, it's about the size of a marble. It's sore to touch and uncomfortable. I have a Dr appt Friday to have it checked. I'm super worried because my 21 year old sister just had a cancerous breast tumor removed and my mom has had the same kind of tumor removed as my sister. Just freaking myself out I'm sure...but what else could it be? I know it could just be a swollen lymph node, but I'm not sick and haven't been. Thanks for any advice!