Is there a pattern to her reactions---bigger dogs, other females ect?

Some dogs go through a stage almost like a baby making strange. Usually happens at about Ivy's age. They develop all sorts of new fears, behaviors, reactions. Most dogs outgrow it and move on. Some will keep those behaviors for life if they aren't handled right.

I'd get her to a class with other puppies. Do you have a Puppy Kindergarten class around? Or a really basic obedience class? It's more for the socialization than teaching her to sit!

One thing that I do, especially with some of my more reactive dogs, is to never let a strange dog approach them directly. I always get between my dog and any approaching dog--even if the approaching dog is being polite and has good doggy manners. My dogs need to know that no matter what I will protect them. They do not need to protect me or themselves. I am the pack leader and will handle those things for them. Franklin used to be just like Ivy--randomly attacked other dogs, especially loose dogs. Since I started doing this, his reactions have decreased substantially. He has no need to worry and react, Mom has it under control.