My 16 month spayed female German Shepherd loves to go to the dog park. She’s normally friendly and playful with other dogs - gentle with small dogs and rough when the big ones play rough, but never aggressive until yesterday. She loves to chase the ball but usually defers to the dog who gets to it first. She has had three incidents in two days of being aggressive over a ball or a stick to the point of attack on the other dog. How can I stop this behavior? I’m thinking of using the e-collar but I’d rather find another strategy. Help! (And please don’t tell me not to go to the dog park!)