My dog is very lethargic

Hi everyone, I'm new here.   I have a 15 month old golden retriever. He's been dewormed earlier this month. He's also recently received his monthly dose of Nexgard for external pests.   About 4 days ago, he ate something that disagreed with him. I still don't know what it was. He skipped dinner for the first time ever that night. He spent the night vomiting (about 7 times). I took him out for #1 that night and he ate a ton of grass and puked it back up.   We took him to the vet's first thing. He had an abdominal x-ray to rule out any type of gut obstruction. There was none, however, they found lots of gas in his lower GI tract. He got some IV fluids, an injection to stop the vomiting, and IV-administered stomach soothing meds.   After returning home, he slept through the day and finally ate some plain chicken breast and rice with a tiny bit of water. No vomiting since. He had diarrhea the next day once or twice. I've included prebiotics and the diarrhea has since stopped, although his stool is still a bit more loose than I'd like it to be. Initially, he was very lethargic and sleepy, not interested in playing or his usual activities, but he was back to being himself the next morning.   The issue is the following: he still sleeps a lot, significantly more than usual. Although he's very active when outside, he gets tired quicker. He has his appetite back but doesn't wolf down his food like he used to. He doesn't ask to go potty or wake up when he needs to potty anymore (his usual schedule is once every 4-5 hours except in the night). He does eliminate normally when I do take him out; he just doesn't ask for it. Urine is clear, temperature is normal, so I'm not too worried about tick transmitted diseases. He's currently laying under the dining room table in another room and he NEVER isolates himself, he always follows me around.   He was meant to have his yearly DHPP vaccine this month, but I skipped it because he's been run down and the last time he got it, he had a fever the whole night and was quite lethargic for days.   Should I be worried? Should I wait it out? What's your advice?

Hello and Im sorry to hear that your boy is poorly, especially at such a young age.  I would certainly reccommend taking this seriously.  I can invent a few stories that might match this pattern, but to find out the truth, your vet will probably have to do an examination and / or tests.   For example, vomiting can make a dog dehydrated and dehydration can affect the liver or kidneys, so even after the vomiting is finished they could still be sick.  2) Some toxin might have caused the original vomitimg, which stopped on its own accord but left damage eg to liver or kidneys.  3)  The vomiting could have been an acute (sudden, severe, out of nowhere) start to what is now chronic (ongoing, underlying) pancreatitis which, even if no longer causing vomiting, could still give the patient a painful tummy and lack of appetite.   4) the vomiting could have caused stress which allowed the onset of some seconandary endocrine condition, such as hypothyroidism.    I'm sure that there are more possibilities;  your vet do a full examination and might decide to check a blood profile, for example.  If there is abdominal pain, they might do further tests eg for pancreatitis.   I hope that something there is useful to you.