If you find a lump in your breast it could be cancer but it also could be nothing. Some people tend to get lumps around their time of the month. Others just have naturally lumpy breast. But either way if you find something that feels different then you are use to better safe then sorry, talk to your physician. Get it checked out. I had to when I was younger thank God it was nothing.
Breasts are fatty tissue with mammary glands to feed a baby, so there are no muscles in the breast itself. If you have noticed something that wasn't there in your previous monthly self-examine, go see a doctor for some peace of mind. Breasts can be lumpy or dense, but any changes should be monitored.