“Breast reduction from size ddd to c”

I had a breast reduction done in Dec 08 and went from a size DDD to C. My insurance covered the procedure. Everything went well, and I had minimal scaring. I exercise regularly and started lifting weights after 5 months. I couldn't have asked for a better outcome.

 I am very pleased with the outcome and the healing went well. However, two weeks ago, I noticed a hard lump under my right breast on the incision line that was very tender. I had a business conference that was a week long, so I didn't go see my doctor immediately. By the time I returned, that lump was black and blue and was the size of a gumball. It was very painful. I called my doctor and was able to get into see her this morning. She cut it open and drained it and said it was full of puss. She has sent off the "gunk" to have it examined. When I asked her what could have caused this after 6 months, she didn't have an answer. I exercise regularly and don't know if I could have caused this by lifting too much weight or if it is a fluke thing.

Do you have any insight on what caused it and if it could be serious? The entire breast is now sore and painful. Thank you.