I had a fibroadenoma in my right breast at age 18 to somewhere in my 20's... It was checked periodically by my GP(s) and I consulted on one occasion with a breast surgeon. It has since been resorbed by my body - gone.
Mine was a similar size and very mobile. They are sometimes called a "breast mouse" because of the way they move about. It's a collection of extra tissue and is nothing to worry about.
Hopefully you have had yours checked out now and everything is ok?