My Dixie has the same thing...she is 4 yrs old now but has had a couple of them since she was about 1. I had the vet check it out and she said to just watch it to see if it gets any bigger. But all this time they have not changed in size. So I just keep a close watch on it to make sure they don't get any larger. Other than that she is a very active, healthy yorkie!!
Kathleen-