Vet was no help, must have been a busy day.
If your pet has a serious illness, I'd suggest taking them to a pet hospital.
I brought in two sick 5 week old puppies, who kept having diarrhea, vomiting, were lethargic, and dehydrated and were kind of "giving up." They were always pretty much active and I was always on top of feeding them formula/mush, and they just got like this one day, despite them being well taken care of. It's the strangest thing, one minute they are crawling everywhere, the next their eyes are closing and become very ill. On my visit, the doctor didn't really run a series of tests to see what might be wrong, just put IV fluid on one of them and tried to see if it would eat... I went home, and puppy #1 died within 12 hours after the visit. The second puppy just died 3 hours ago. After deep research based on the symptoms these puppies had, they definitely developed parvovirus. I kept asking the doctor if there's a better way to keep the puppies hydrated, and he said no, and that puppies sometimes just "give up" as if there's no logical explanation. Honestly, I think this has been the strangest experience, even at a vets office. Animals don't just stop eating for no reason. Puppies with parvovirus usually don't die from parvo, but die from dehydration. Every half hour the the formula/water would just disappear from their tummies when tube fed to them. They would either vomit it, or diarrhea. So there's no winning unless I had an IV fluid kit and antibiotics.