Help determining type of reactivity

Hello! I have a 9 month old mixed breed rescue dog. She is great with our 3 other dogs, and loved other dogs until around month 7/8, but she is now extremely reactive to other dogs. I believe her genetics are a contributor, as she is mainly a herding/bully mix and loves to herd/chase/gets overstimulated. Other than that, there have been no specific situations that would have caused this.

I am having a hard time determining if it is excitement based reactivity, fear based reactivity, or leash based reactivity. Any guidance would be appreciated.

I am confused because she pulls towards the other dog, which I thought was a sign of excitement based reactivity, however once she gets to the dog (on leash) she doesn't deescalate, and she acts aggressively, snarling, barking, snapping.

She recently completed a weekly puppy class. Some times she was able to have calm, friendly, on leash interactions, other times she acted aggressively. Lunging, barking, snapping. It confused me even more because these were the same dogs that some days she'd be okay with and some days she'd be aggressive towards. I do want to note that there were two dogs in the class that she never acted aggressively towards, both her size/smaller than her. And she was able to calm down and sit/lay near the other dogs in the class frequently.

I also wanted to note that one day during this class, she wiggled out of her collar and ran up to one of the dogs she was reactive too (though not completely losing her mind at the time) but once she got to the dog she did nothing and calmed right down.

I am worried it is getting worse, as I tried to take her for a walk with her sibling dog and a friend's dog that she had met twice before and as soon as she saw the friends dog she lost her mind, barking, lunging, snapping.

I just feel like if I could determine exactly what this is, I would have an easier time working on her with it. She doesn't have to like other dogs, but I need her to be able to be around other dogs without losing her mind.

Thank you!