I picked up a mix from the pound a while ago and he was a lovely dog. a bit shy but was never people or dog aggressive at all. However I recently too him to the dog park where a German Shepard went aggressive. She rolled him over once and pinned him down and the owner got a hold of her. About a minute later he lets her go again and, again, she rolls him over and this time gets a grip on the scruff of his neck. Now after this ordeal, my dog is terribly shaken and is growling at any dog that comes near. I calm him down and after about 5 minutes he's back to playing. HOWEVER, now every time a dog shows dominance (for example by placing their head over his back or something) he spins around and straight out attacks the dog. He's never done this before until after the GS. He's gentle & extremely playful with them until another dog walks up to him and shows dominance. I have a feeling this all has to do w/ his GS experience. Is there a way I can help him come out of this trend?