My dog hasn't been eating, loss of energy, and throws up yellow stuff?

i have a 3 month old border collie and all of a sudden she just stopped eating her food and even her treats. she had diarrehea also and she has not been as energetic as she use to be so i immediately took her to the vet. after a day of diagnostics, xrays, and tests the bill came out to $1000 via Petsmart Bansfield Veterinary Services and they concluded that she has a stomach infection. i just want to know if this is possible as i dont want to get taken for a ride