how do u know that u have breast cancer?

The best way to confirm is go to a doctor. Even if you are not sure just go for it. Early detection is good for treatment.

You wouldn't know unless you go to someone that knows how to diagnose it and the only person I know is a doctor. I suggest the earlier it is for you to have it check the better. You may start consulting with your obgyn doc to have it check if your uncomfortable with a surgeon. If your ob does find something that is to be considered unusual she will refer you to a surgeon to have it checked and be mammogramed and if they find something in your lump then you'll have to have it removed surgically or else it would spread fast if not taken as early as possible. The lump may also be a non malignant cyst which sometimes dissolves in time. You may have these when your having your monthly periods. You may opt for it to have it surgically removed if they are bothersome to you or have it there if it doesn't trouble you. But overall these you have to let it be checked by a doctor no matter what. Prevention is the best defense for it.