Australian Shepherd Is Suddenly Aggressive—PLEASE HELP

Hi All, I have an almost 1.5 year old female Australian Shepherd who started showing possessive behaviors at around 7 months right after she got spayed. It started with her toys and then even like sticks or the water bowl at the park she would snap at other dogs.

I worked with her to try to remove these behaviors but they never went away. Recently, she ran up to someone who was playing fetch with their dog, sat and waited next to their dog for the ball to be thrown and before he threw it, turned and attacked the other dog in anticipation.

I was shocked, and stopped letting her run free where other dogs were present because I couldn't trust her and didn't want to see someone else's dog get hurt.

Yesterday, I traveled for work and left her to be boarded at a new facility. Very nice place - the guy turned his home into a "home style" facility. The dogs can play all day in the yard and hang out inside or out. I got to my hotel to three emails from the place because my dog had attacked one of the owner's dogs when the owner's dog tried to get on the couch near my dog (Sadie). She left puncture wounds and had to be PULLED off of the smaller dog. I am in shock and devastated.

I got her at 9 weeks from a breeder who still has two of her siblings without issues. She was well socialized with people and dogs from a young age - in fact several people in my apartment all got puppies at the same time and they had an almost daily play group. She went to daycare for months without any issues and has been through 18 weeks of general obedience and since then has been in an agility class for almost a year now.

Does anyone have any idea why these behaviors have developed and what I can do to help fix this? I am completely overwhelmed by this most recent occurrence. Could it be a medical issue, something hormonal? I did see a behavioral change after she was spayed where she seemed more nervous/fearful of things (including harness that she had worn for her entire life). She had thyroid panel done at age of 1 year and it was fine, but I wonder if I should have it checked again. Are there any other things I should have checked out? Any other suggestions in general?

Thank you in advance - I love my dog and am terrified by what is happening.

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