Sorry to hear about Roxy. No fun when your dog doesn't want to eat! Any vomiting or diarrhea? Did she eat anything different on July 4 that might have upset her stomach?
Besides the injection yesterday (do you know, was it Cerenia or something else?), is she taking any other medication? Did your vet just think it was gastroenteritis?
Hang on a second while I look up daffodil bulb ingestion.
Ok, daffodil bulbs (and tulips) can cause a pretty severe gastroenteritis, so that may be the problem.
The Cerenia was a good choice to help control the vomiting and nausea. Since she is still feeling poorly, and especially since she isn't drinking today, I think it would be good to take her back to your vet for SQ fluids (under the skin) and another injection of the Cerenia.
Hopefully just time and supportive care will do the trick. Daffodil bulbs are quite toxic and can cause convulsions and low blood pressure, but it doesn't sound like that is a problem. So hopefully just another day of fasting and supportive care and she will start feeling like eating in the next 24 hours.