What Causes A Lump On The Sternum?

3 days ago noticed marble-size lump to left of sternum. I thought it might be sabaceous cyst or boil, however now it is the size of golf ball. It isn't red or infected looking just hard mass. I saw Dr. today he tried to drain it to see if it was an abscess. There was no pus or signs of infection present. The Dr. also inserted a q-tip about an inch into the incision and also tried to aspirate any fluid. To our surprise, there was no pus and only serosanguinous fluid present. Minimal drainage. The Dr. placed me on Keflex and said if it isn't better in 2 days to notify him and he will refer me to a surgeon. Any thoughts as to what this might be. Of course I have been researching articles on the web, and now I am scared that it could be cancer. Although, I though most cancers don't grow that big in 3 days. Any suggestions on what I should do? I do have history of lipomas and fibrous tumors. I do smoke about 1/2 pk per day. I do not take any medication on regular basis. I am 40, 5'8 and weigh 175 lbs.