Could well be a fibroadenoma. Also known as a 'breast mouse' because it feels a bit slippery and like it's moving around slightly under your fingers.
I had one about 16 years ago, as big as a large grape. Located very close to the nipple, in the left hand side. Nothing to worry about. I had it removed and you can't even really see the scar anymore. Just a faint silvery line.