Hello LucyGoose - Strange. The exact same thing is happening to my dog Maggie right now, but she is 14 years old. She won't eat, very listless and her front leg seems to be giving out. It is a great effort for her to even go outside and for 15 hours straight she WOULDN'T go out. She does get people food once in awhile but I don't know which ones are not good for her. She doesn't get any candy but has things like baked potatoes with skins, etc.
I did have an extensive blood panel done yesterday and found out tonight all her vital organs are good except she has extremely low
thyroid (needs to be back on her meds which doc had us quit in September) and her white blood cell count is up so there is an infection somewhere. Vet is starting her on an intense antibiotic program tomorrow as well as giving her some more thyroid medication which she shouldn't have ever stopped. The reading was so low it didn't even register on the thyroid.
I suggest that you have some blood work done, if you haven't, especially the thyroid. When I read your post you could be describing the symptoms of my dog. And yes, like yourself, I kept asking the doctor if she might have the flu. Hope everything turned out okay as I see your post was about 10 days ago. I know how much our pets mean to us. Let us know.