What side-affects to tell your dog has stomach flu
sad puppy

Dogs don't get the stomach flu. They get Canine Influenza, but it is mostly respiratory than GI.

If your dog has a painful stomach it could be a number of things.. listing from emergency to minor

The emergency factors are Gastric Dilitation Volvus or just Gastric Dilitation. The stomach fills with gas and can twist inside the body. It is extremely painful and life threatening and requires a vets attention NOW.

Impaction is also very painful. The dog eats something that cannot pass through the intestines. The rest of the body is trying to eliminate whatever is in the intestines, but the intestines aren't strong enough to push such a big object. Another life threatening problem that needs a vet NOW.

Bloat.. which is a lesser form of Gastric Dilitation.. Still very painful and can turn into Gastric Dilitation very quickly.. Another thing that needs a vet

Allergic reaction to something the dog ate. The insides swell up and nothing can pass through. The stomach is overly extended with food and the dog is in severe pain

Gastric ulcerations.. Painful and requires a vet

Worm overload. If the dog gets overloaded with worms, I would think it would be uncomfortable. Overloads of worms can kill dogs.

Has your dog been hit by a car that you know of? Stomach/abdominal pain can occur if the dog was hit by a car, kicked by human/other animal...

Consitpation.. I know I don't feel good when consitipated, I'm sure dogs don't either. Not as serious as the others.. but still in need of a vet since we don't know what exactally is going on and why.

In conclusion, you need to go to the vet. Bottom line.. no way around it. It could be nothing, or it could be a matter of life or death.