Second opinion on lethargic puppy?

My 21-week-old border collie mix started acting lethargic and uninterested in food around 2 days ago. She has not cried to wake us up to go to the bathroom (we wake up on our own) like she normally does and ignores her breakfast and lunch. She was very hot to the touch when waking up 2 days ago. Totally normal bowel movements, no vomiting, and will play with her dog friend but is not as interested in playing otherwise. She got a burst of energy last night and had a normal, alert evening walk and wanted to play with a tennis ball inside at 11pm. However, this morning, she was a little better - went to the door to go potty and ate half her breakfast after the vet. She had a low fever (103.3) at the vet today. She has a habit of carrying rocks around, so they scanned her GI on the X-ray, but nothing showed up. Vet said she may have eaten a rock and already passed it, possibly that she's just irritated and recovering now. They gave her fluids, anti-nausea shot, and probiotics. She perked up a little after the vet, but is still very tired. Just looking for another opinion of what could be going on, I'm worried about her!

Hello - first, to give you the boring but essential speil:  I cannot possibly give you a second opinion.   Second opinion implies that the second vet has something to add to the first vets' expertise, by that experience or better imaging or a specialism.  I have less information than the vet in charge of this case,who has at least examined the patient, so I am in no position to give a second opinion.   However, do I have lots to say?  Aways!

It sounds as if your dog is female, about 5 months old with fever and lethargy.  Your vets elected to radiograph first, in case of an obvious foreign body, but haven't found one.  So they are treating the symptoms to look after her and see if they go away, while planning what to rule out next.

So what else could be causing lethargy in a young female? Well, there may be lots of possibities ("differentials").  I wonder about false pregnancy or the onset of reproductive signs, even a pyometra (womb infection) which are rare but not unheard of in youngsters.  Similarly, pancreatitis (causes abdominal pain, usually), sore joints, anaemia (low red blood cells), kidney disease, liver disease, joint pain, autoimmune diseases, and a whole host of others, some of them quite minor. Your vets will likely have thought of these and more; indeed, their list will be tailored better to the identity of the patient.  It sounds as though they might have decided to rule out obvious foreign bodies right away.  Its not a problem if they don't have a diagnosis yet;  what is important is that they can rule the most serious things out quickly enough and give treatment to keep your pup safe while they get to the bottom of the mystery.  You don't mention blood tests, but these might play an important part if the condition continues.  Best of luck - please will you let us know how things go?

It certainly sounds to be important to keep revisiting your vets until things are better.  They should tailor treatment towards her and may offer blood tests or ultrasound, if they haven't already, to find more information.