Quote:
An allergic reaction this far out is extremely unlikely. Besides, you rarely see hives with food allergies. Usually food allergies cause itchy skin, particularly around the face, sometimes weepy eyes, and often diarrhea or soft stools. But it wouldn't cause stiffness or swelling in the neck.
Kitten food however is high in fat, which could trigger a bout of pancreatitis which could account for the lethargy and lack of appetite. Pancreatitis can be triggered by high fat foods, particularly if the dog is not accustomed to foods with a higher fat content. Pancreatitis is much more serious than a simple gastritis and really really needs to be treated by a vet right away. It can be fatal if not treated.
Pancreatitis still wouldn't explain the lump on the neck. But then again, it is entirely possible that whatever is going on with his neck is totally unrelated to whatever is causing him to be lethargic and uninterested in food.
Regardless, this sounds like a dog that needs to see a vet ASAP.