It sounds like a hot spot may be forming to me. Are you sure there isn't a tiny bite of some sort on there? It could be related to allergies. I know some dogs lick and chew out of boredom, but I wouldn't think that was the case with Rosie. Also, some dogs lick a spot that is wounded and are very protective of it.
Since this is a very abnormal behavior for her, you should probably have the vet check her out just to make sure everything is okay. That happened to one of my parents' Old English Sheepdogs and one of her toes was out of place!
