My papillon, carly has always enjoyed and played well with other dogs, until recently. Now when I take her to the dog park, she is sort of mean to the other dogs. And if there are balls there, then she hoards all of them (if another dog tries to get one, she shows her teeth to them, too). this is a new behavior, and it makes me upset, because now I don't take her to the park, in fear she would act worse. She is great with people, I mean super friendly, affecionate, and really loving. How can I stop this behavior? thanks for any advice!!!