Oh no, sorry to hear about your dog. Is the fast breathing an new situation? It sounds like you were at the vet recently? Why do they say this is happening? What about your dog's tongue color? Is it pink or darker? A darkish color might be a sign that she isn't getting enough oxygen. But if she had gastritis, she may just take a bit of time to feel better./ Try bland food like boiled hamburger and rice. And really, what is most important is hydration. Is she drinking water?
Pink gums are good! I know that when my dog had an incident in which she got into children's Tylenol which can be lethal, after we got home from the vet hospital, she would breath funny and kept sticking her tongue out. I called the vet. The vet told me that it was probably normal and I was just now noticing it. It may be something like that. Hopefully with the meds as they are gone, she will get back to her usual self. Keep her hydrated as you are. Let me know how she is in a couple of days. She hopefully will begin to bounce back. Poor puppy!