Drooling and stress
It sounds to me like a sign of anxiety. While your dog has been around other dogs, this was the first time at the off leash dog park. A new place comes with all sorts of new smells. Plus, dogs who aren't on leash can give different signals than dogs who are leashed and accompanied by people.
I'd continue to go back, walking your dog outside the park or simply sitting outside. Take the introduction to the space slowly until she appears more comfortable. The next time you go, it won't be the first time. Her reaction may be different.
Cedar (tri), Willow (blen), Holly (ruby), & Bella (blen)