Thank you for the additional information. The main reason I asked about an ultrasound was because I was concerned about pancreatitis. His labs don't necessarily indicate that is the issue, BUT some dogs can be symptomatic (vomiting, diarrhea) before there are obvious differences in lab work. Given the elevation in the ALT, I was thinking he could be experiencing an anaphylactic (allergic) reaction. The red welt you saw could be from the insect that caused the reaction. Perhaps most frustrating about this kind of reaction is that you almost never find out the exact cause. In most cases, however, that does not hamper the treatment because it is supportive care (iv fluids, steroids, antihistamines and other medicines to address nausea and diarrhea). In severe cases, clotting times are elevated and plasma transfusions are needed. Clearly your dog was not experiencing life threatening symptoms, but was still pretty sick. One other aspect of your description that made me consider anaphylaxis was the fact that his symptoms improved and worsened. This is pretty classic in allergic reactions as the body struggles to fight the overwhelming histamine response to the initial insult (bug bite, environmental toxin, food source, etc). The following websites will give you some information on anaphylaxis, and anaphylactic shock.
I suspect that the grogginess you are seeing and the fact that he tires easily is due to whatever is going on with him and is not due to the medications. The symptoms you describe are atypical for the medications he is on, but it is possible since dogs are like people in that they can be sensitive to different things that would not phase another dog.
The following website will give you more information on metronidazole.
The following website will give you more information on Tylosin.
The following website will give you more information on Zentonil.
I think it is worth it to discuss this possibility with your vet. You may also want to inquire about starting some anti-histamines and should consider keeping a supply at home. Another awful thing about allergic reactions is that they can recur, and sometimes each reaction is a little worse :(. Benedryl is a common antihistamine that can be used in dogs, and it is given at a dose of 1mg per pound of body weight. So a 25 pound dog would get 25mg of Benedryl. I would buy the tablets and not the capsules so you can cut them in half if needed. It's better to slightly under dose than to overdose. Benedryl will need to be given every 8 hours, and it will cause drowsiness initially. If this happens again, take him to the vet ASAP, like you did this time. You were smart to take him in, and if he was dealing with an allergic reaction, your quick action saved his life. You can speak to your vet about an Epi pen if he does have another issue, but hopefully you won't need to think about that.
I hope this helps, let me know if you have further questions. Good luck with your dog, I hope he's back to his normal self very soon.