Sorry you and Cassie didn't have a good vet visit It does sound like fear aggression rather than a dominance issue. It sounds like she showed a classic flight or fight response.

Unfortunately of my four dogs, two are fear aggressive.. my rescue Sheltie and my adult Malinois. I've paid thousands for dog training and behaviorists and fortunately have learned enough through experience to manage a fear aggressive dog now. I'm happy to say that my rescue Sheltie has been fully rehabilitated and my Malinois is well on her way.

The worst thing you can do is correct aggression with aggression (verbal or physical correction). I would suggest desensitizing at a pace Cassie is comfortable with.. however slow that is. It's important you don't let her get over her threshold. To give you an idea, I'll explain what I do with my dogs. Keeva didn't like strangers coming up to her. She'd flip out if they got to her level and tried to pet her. So whenever someone came over or we were out and someone wanted to say hi, I would explain our situation and ask that they just throw a treat. First it was about 10 feet away before she started hitting her threshold. We kept at it and slowly she accepted closer distances. Now she'll take it straight out of someones hand and accept a pat on the head. We also do focus exercises when we're around strangers or dogs (she doesn't like dogs either). Same thing.. she had to be far away enough to notice the dog/person but not be freaking out. Everything has to stay positive. We compete in agility with tons of people and dogs so the focus exercises have been great for us. We've never had a single incident in agility.

Good luck! PM me if you'd like more tips or have any questions!