I agree with Marie. I too believe it’s a neurological disorder. You should, though, have her thyroid levels checked just to rule that out. I once had a Tibetan Terrier, Tracy, that was as sweet a dog you could ever want, also at the same time a Maltese (she’s now 14 1/2 years old). They were best buddies, played together, slept together on our bed, loved each other. Then one day for no apparent reason whatsoever, and trust me, I’m very observant and a great detective when it comes to figuring out why my dogs do what they do and where and what they’re up to at all times, Tracy went after Hannah. Grabbed her, started shaking her and scared the crap (literally) out of Hannah. Hannah was unhurt thank God but at that point Hannah was terrified of her. She was very careful to take the long way around a room, behind furniture, etc. if she wanted to go anywhere. Tracy also was in wait trying to attack her. It was a horrifying situation for all of us, obviously, most of all for Hannah but I have to say that I always felt that poor Tracy seemed like she didn’t know what was coming over her and what was happening. Finally at the age of 10 she was diagnosed with Neuronal Ceroid Lipofuscinosis (NCL). Common signs of NCL is the development of aggressiveness toward other dogs and sometimes eventually, but not always, people. She was diagnosed after seeking help from a neurologist. Tracy finally succumbed to liver cancer which I was later told could also have contributed to her developing aggressive behavior. NCL is a neurological disorder that is prevalent in some breeds. It breaks my heart to have to tell you that Corgi’s are one of those breeds. There is no cure. It is what it is. If you google the disorder in canines, there is a site that will give you the list of the breeds. I believe that you are dealing with NCL.
