She also has a small tatoo on her belly, its a circle and a cross below, the sign for female. I don't know if that was cosmetic or a breeder thing, but its something I've never seen before on another dog.
My guess is that her tattoo is a sign that she's been spayed. I've heard of female dogs that were spayed at some humane society type places being tattooed so that if she winds up in a bad place again, the people taking her in will know she's been spayed. The ones I've seen have been an X in a circle. Saves doing another surgery.