I am sorry to hear that you have a lump in your breast that is firm and that you are worried that it might be a cancer. I am happy to give you some general information about breast cancer, but ultimately I am going to recommend that you make an appointment to be evaluated by a breast surgeon
(which is a general surgeon that has specialized in doing breast surgery
). The reason for this is that they will be able to ask you more questions (i.e. take a thorough history) examine you, and take a look at your records to give you the most accurate opinion about what the next best course of action is. It sounds like you have already gotten an ultrasound of your breast, which is a good first step in working up a breast nodule. As I am sure you are aware, there are many reasons to have a nodule in your breast, the most worrisome of which is a breast cancer. While the fact that it is 'harder' than the tissue around it on an ultrasound isn't necessarily a good thing, it doesn't mean that it it for sure a cancer. The only way for someone to tell you for sure if you have a cancer, is for them to either take the entire thing out, or take a piece out of the mass (called a biopsy) that they can send off to a pathologist
to examine with a microscope. This is why I would recommend that you are evaluated by a breast surgeon. Best of luck.