It isn't a good idea to not have your dog vaccinated but it is believed that vaccines actually last more than just a year or two, so while we can't rule out a disease, I think it is more likely to be a different issue such as pancreatitis or a disc issue.
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. If you have fed your dog any fatty foods like bacon, ham, fried foods etc, in the last few days, that might trigger pancreatitis and your dog is overweight as well.
Symptoms of pancreatitis vary but 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. They will frequently not eat with this. This sounds like it could be a possibility. If he starts vomiting, then I would feel pancreatitis is an even better possibility. You can read about it in detail here:
Chihuahuas are a long backed breed and they are prone to disc issues. Dogs with disc issues often react to the pain with an arched back and they frequently will avoid stairs and jumping up which also fits your dog's symptoms. He may have injured it playing at the park or even slipped or twisted wrong today. You can read more on disc issues here:
Buffered aspirin can be given to a dog with a dosage of up to 5-10 mg per pound every 12 hours for pain. Keep in mind that a dog's body does not metabolize aspirin in the same way as a human and thus should not be given more than a day or two without contacting your Vet. The aspirin may need to clear your dog’s system before other medications can be given, so keep that in mind if you decide to give aspirin and be sure and tell your vet when your dog is seen. Read side effects and precautions here.
Often a constipated dog might also exhibit symptoms like you are seeing as well. If he has eliminated recently then that shouldn't be the issue. He would likely be attempting to eliminate but you state he isn't.
Since pancreatitis is considered an emergency and a disc issue is best treated early in order to keep a dog mobile, I would have your dogs seen by your vet to determine which is the most likely cause especially since it is a weekend tomorrow.
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