6 year osteosarcoma surviver, with new lump on breast! Help!

Hi guys, I am a 25 year old osteosarcoma surviver.  I was diagnosed in 2007 and underwent surgery and chemotherapy.  My primary site was in my right mandible, and it never spread.  Last night while showering, I felt a large mass in my upper right breast, and I am freaking out! I contacted the hospital and should be able to get in to see someone in the next few days, but I am going crazy thinking the worst.  Has this happened to anyone else out there?  How did it go?  Thanks for your help.