Thanks for the additional information. The position you indicate with the rear up in the air is generally considered a symptom of abdominal pain. This is frequently seen in a dog with pancreatitis. Pancreatitis is an inflammation of the pancreas. The pancreas is a gland that produces and releases enzymes into the digestive track that aid in the digestion of food. When these same enzymes are activated while they are still in the pancreas, inflammation occurs and the enzyme start "digesting" the pancreas itself.
Some common causes of pancreatitis are eating a lot of very fatty foods, obesity, steroid use and other diseases and conditions. In some cases, it just happens. Symptoms of pancreatitis vary but can include vomiting, diarrhea, anorexia, weakness or collapse, dehydration, shock, fever, depression, and upper abdominal pain. A dog may indicate abdominal pain by acting restless, panting, crying or wincing when picked up, shaking, standing with an arched back, or lying with the front end down and the rear end elevated.
You can read about it in detail here:
The fact that he does have diarrhea and may have vomited do point toward either pancreatitis or a possible gastrointestinal issue. Pancreatitis is considered an emergency. I would urge you to have him seen by a vet as soon as possible. In the meantime, you may want to withhold food and limit water for 6-12 hours then offer very small portions of boiled chicken and rice. Start with just a teaspoon of food and if he does ok, give more in 30 minutes or so. Gradually increase the amount if he can handle it with no vomiting or discomfort (indicated by either panting or his position). If there is no improvement after that, you will need to have him seen.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.