Can you take her for an off leash romp somewhere safe? See if the leash and collar have somehow triggered some bad feelings? A young dog that doesn't want to go for a walk, is going through something strange, indeed.
Continuing to force a walk when her mind is in such a state can't be helpful. Does she love the ball, fetch, or some treats? If so, maybe you could snap her out of it using those. I'd try to find some activity where she shows no fear and is confident and happy, and engage and reward her for this, and see if you can build up confidence before trying a walk again.
How could the vet tell that it is NOT a split heat with a physical exam? They aren't always obvious. Did you progesterone test? Neither are false pregnancies, even with blood testing. I can't imagine you could even test for one, it would be based on the dog's behavior over the weeks.
My recommendation, if it may be hormonal, is to spay her at about 2 years because hormones can have MAJOR impact on a bitches' behavior. It depends on the individual dog, but I've seen major temperament changes in pregnant dogs and dogs in heat.