Hi Aimee! Nice to see you again. How are things otherwise?
It was some years ago, but I remember another child on the forum that the same thing happened to. I think she collided with the dog. I do believe it resolved itself, just like any bruising would, but you would be better to ask someone else than me.
I would ask Dr. Levitin: http://www.birthmark.org/node/152
Good to see you, though! Wish it were for something more positive.