I can't say what may be going on.. but certainly call the vet back if there is no improvement.. like tomorrow. A bad tooth or something in the mouth was my first thought. I have a chihuahua, too and I couldn't take it if he was doing that. Did your vet say your dog was having a seizure? Was his temp high?
I read where you took her "to a vet" but all they did was check glucose levels and give phenobarbital. That sounds more like an "Emergency Animal Clinic" visit. -- Was that it? If so, then ALL they were going to do in her case was make sure she was not in danger of needing emergency surgical or respiratory care. They just tried to keep her stable -- they do not diagnose, they do not treat, they just keep animals in crisis stable long enough for you to get the animal to a regular vet's office the next day for the actual tests and diagnosis. -- That's the main purpose of the emergency service clinics. -- What you are supposed to do now is take her to your vet and get full tests and examinations done! -- If the person you already took her to was a regular vet (and not just an emergency clinic), then you need to take her to a different vet!