Dear Carol in Pittsburgh: Surgery causes trauma to the tissue and there can be many possible variations to the healing process, including hematoma, excess fluid buildup and infection, to name a few. Some of these will resolve on their own, in time. Others, infection, for example, may require intervention. You should let your surgeon know what is happening. He/she will be better equipped to make a diagnosis and appropriate recommendations.
It's very common for a lumpectomy site to become swollen and hard. It often gets quite black and blue, as well. Usually the hardness is at its worst around 3 or 4 weeks, and can take a few months to soften up. But, since it can't be seen by an online answer, it wouldn't hurt to let your surgeon know what's going on.