Bring her/him to the vet. Maybe it's the quality of the food or maybe she/he ate scraps from the floor or too much treats or food. A good type of dog food i suggest is Origin. Is #1 dog food in the world. Its all natural containing no wheat which is bad for them. It has 70% protein and the meat is in the kibble was caught fresh not frozen with tons of artificial things. All the dog food in groceries stores or petsmart or petco have wheat in them and as little as 21% of protein at the most or less. You can only get origin in dog boutiques. It will be life changing for your dog.
gastorenteritis is a very dangerous condition and can actually cause death in deep chested dogs, i.e. dashchunds, greyhounds are just two off the top of my head. It cause`s what is called bloat! Try not to feed your dog large meals and not from the table. Also don`t "free feed", let them eat what they want for 10 to 15 minutes then put their food bowl up. You can give them, pepto bismol, I had one little doxie and she was always getting sour stomachs, I always called her pepto, when I said that word, and she saw that pink bottle she ran, lol. Also check with your vet, you can also give them tagemet.