Lethargy and whining - 18 month old dog

Hi, hope someone can help me more than my vet can. I’m worried that my 18 month old border collie poodle cross is suddenly very weak and lethargic (not sleeping more but constantly needing to lie down) for context he is extremely active and playful but now shows almost no interest in toys or any sort of play. On the odd occasion that he instigates play like he used to, he does not end up running after a ball at all or runs after it and then lies down. He is also crying a lot, and stands in a sort of hunched position which suggests pain to me. He has also lost around 1kg of weight. This started around 2 weeks ago and is only getting worse. He isn’t limping or licking his legs though. It’s important for me to mention that he has had various other health issues in this time period. First of all around a month ago we noticed he was licking his penis a lot and crying more, and we had his urine tested for a UTI but both local tests and lab tests came back negative. However the vets did give us antibiotics just in case as he had the same symptoms last year with a UTI. The antibiotics don’t seem to have worked and he is still licking it at every opportunity. Also, around a week ago he had a day of vomiting, he threw up a total of 8 times and was taken to the vet who did blood tests on him (no pancreatic issues, no anaemia and no toxins) and also checked for obstruction but found nothing. He was given a suppressant and hasn’t been sick since. I mention all of this in case it might be connected to the lethargy and lack of interest in play. If anyone has any ideas please let me know! My vet no longer believes us and thinks he is doing it for attention but I know my own dog. Thank you

Hello - so your dog is weak, lethargic, standing hunched as though in (abdominal?) pain, and had a whole day of vomiting about a week ago.  When did you last see your vet?  If you havent seen them since the most recent signs, it is almost certainly a sign that your dog needs to be seen - and they should be phone-triaged as regards to how urgently.   I am surprised that you say that your vet doesnt beleive you; have they actually said this?  Sometimes I find that the message a vet means to convey and what the owner picks up can vary massively, so I find that the following questions can usually get things back on track:  1) do you beleive the symptoms that we've reported?   2) What things could usually be causing those symptoms?  3)  which of these have you managed to rule out so far?   4) what is left to rule out?   4.5) have you ruled out pancreatitis (this is by no means the only possibility or even necessarily the most likely, but I like to mention it as it is generally underdiagnosed in dogs of all ages).   5) what course of action do you propose going forward?  What about to control the pain?    Its sometimes a good idea to have your questions in a list so that you can take notes.   I hope that something here helps.