Thank you for all the suggestions. What I'm noticing is that this only happens with dogs she doesn't know. We have another Shih Tzu and a cat and there is never a problem. We are snow birds and often see folks from the previous year and Cookie has no problem with dogs she met last year, even with one she only met once the previous year. In Yuma I found that there was no aggressive temperament if she was off the leash when we walked along the canal, if other dogs approached she would move directly behind me and go into her shaking routine.
We are in Tucson, AZ and I will have to see if anyone can steer me towards a good Vet.