My 6 month old staff puppy is great with other dogs, my bofriend has to lurchers and a jack russell and she loves them. However, when shes playing with the jack russell and shes on top of her nipping her and doing what pups do, she starts really growling and i have to pull her off.. Why does she do it!? Is it over excitement? Or is it dominance?.. Shes never bit her its just a load of growling but it sounds horrible.. how do i get her out of it? Any ideas?