Now I am quite concerned about Snickers. When dogs have tight bellies, this is often a pain sign. It is something we can see with harmful ingestions (which I am glad to see isn't' likely), but also with severe bacterial or viral gastroenteritis, and pancreatitis. The last one could be triggered by a food change. As well, while less likely if this has suddenly come on, we can see gut thickening, tumors, and IBD cause signs like this; as well we can see tightness from enlarged organs or fluid taking up more space in the belly then it should. I do feel these are less likely for Snickers but they would be concerns to keep in mind at this point and if his belly has been hard for a while.
With this all in mind, for the moment, we can try to soothe his gut to see if we can settle these signs. For that, you can try him with an OTC pet safe antacid [ie Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac)]. Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned. Though if he cannot keep it down due to nausea, that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Afterwards, you can consider starting an easily digestible diet like cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset. As well, if he seems gassy (which can distend the gut), we could treat with Simethicone (1mg per pound every 8-12 hours), add probiotics (ie Benebac, Fortiflora) +/- add activated charcoal to his food.
Overall, Snickers's signs do raise some concerns and with the discomfort he has (the cry when lifted does suggest abdominal pain), we do need to be careful here. So, as long as his gums are normal colored for him and you dont' think he ate anything harmful, we can try the above now. But if this lingers or he is very sore with that belly, then we'd want to get his vet involved. They can assess his hydration, rule out fever, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with injectable anti-nausea medication, gut safe pain relief, gastroprotectants, +/- antibiotics to get him back feeling like himself.
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