I doubt it's the food. Even though Royal Canin isn't highly regarded..lots of Bully's do fine on it. She should be protected well enough from Parvo after 3 vaccinations.although, I wouldn't take my pup out and about until they have had all their shots.....Is there a chance she could have eaten anything else? A piece of toy or anything outside? It is possible she just has an upset tummy, but honestly, if she were my pup, I would take her to the vet. They can get dehydrated very quickly. I will tag a couple people who might have more advice for you. I know you are worried, we all worry for our sweet Bully's.