sudden aggression

Hi Everyone :) Sorry - it's a long post! Wanted to include as much information as I could! We just adopted our 1-year old German Shepherd x Golden Retriever (they think) mix 1 week ago. She is generally a very VERY sweet girl. I would say a solid 90% of the time she is every dog owner's dream...

My 1-1/2 year Beagle mix is very socialized. Been going to dog park for over a year, perfect with 3 kids, likes strangers and other dogs. Suddenly is acting more tentative and nervous and when we went to the dog park this weekend - twice - first acted snappy when a dog came up to the kids...