I'm a dog trainer and I work with dog behavior instead of being a veterinarian. I am a Certified Pet Dog Trainer and have a lot of experience on the behavioral end of things - so that's the perspective I will have to take here and will have to suggest you also seek advice from a veterinarian on Just Answer to get a veterinarian's viewpoint.
Thanks for providing such good and detailed answers to my questions. They are very helpful.
In short, a panting and trembling dog can be due to anxiety/fear or due to the dog actually not feeling well.
Because the one time in which he displayed the panting and trembling behavior in the past, he was not feeling well, and because this bout of panting and tembling coincided with him having a sinus infection, I'm really leaning towards the panting and trembling as being related to him not feeling well. It may have started as being due to the sinus infection itself - and now it's either still due to the actual sinus infection or due to some other reason he's not feeling well.
Furthermore, I don't think the panting and trembling are associated with an unrelated anxiety because you told me that he normally is happy go lucky and does not normally display fears or anxiety.
Furthermore, he is not feeling up to par during the day and he is not eating as well as before (even though he is a bit of a finicky eater anyhow) - this all leads me to think that the trembling and panting are due to something physically bothering him.
That's great that his sinuses are clear! When you gave him the aspirin when he started panting and trembling, did you notice any improvement? I'm thinking in terms that the aspirin would not only help to lower a fever, but would also help with pain. If the aspirin did not help, that makes me think that maybe the trembling and panting at this point in time are not due to a fever or sinus pain (but I may be wrong - just thinking out loud, because maybe his sinus' are still bothering him even though the infection has cleared up???) Wonder if his nasal passages/sinus' are feeling dried up and still painful even after the infection appears to have cleared? (this would be a good question for a veterinarian).
On the lethargic, not eating as well as he used to, eating feces, and panting and trembling in the evening side of things, I'm wondering if his stomach is topsy-turvy? I have had to treat my dogs for Lyme disease with antibiotics (and also have had to treat myself with the same) and I can attest to the fact that antibiotics can do funky things to a digestive system (I don't think "funky" is a veterinary term!!) - but, I do think that something that would be worth a try is probiotic supplementation. A funky digestive system could be suspect in a dog that never ate feces before and all the sudden started to eat feces (plus he's not eating as well as he used to). Furthermore, aspirin could be an additional suspect in a stomach that is not feeling up to par.
Do you think that the Pepcid AC is helping any? I would also try a probiotic supplement (i.e. acidophilus or check for digestive support supplements available at http://www.kvvet.com/) to see if you see any changes - note that a supplement like this would take time to work (i.e. the difference won't be immediately apparent). I always recommend consulting your veterinarian prior to giving your dog anything new.
Plus, you should keep working with your veterinarian as long as your dog continues to pant and tremble in the evenings (ask your vet to keep working on the problem).
When my dog Blue was sick he would pant and tremble at night more than in the daytime. I think that it was because when he got tired at night the pain bothered him more. Also, if something like a sore muscle is bothering me and I'm being active during the day, I don't notice it as much. But when I settle down at night, then my mind is on it and it bothers me more (the same would be true for a dog).
Sorry I cannot pin-point exactly what is bothering your Lhasa-Poo. But I do think that something is physically bothering him. In the beginning of the post I said, "a panting and trembling dog can be due to anxiety/fear or due to the dog actually not feeling well." I'll add a third possibility here: The panting and trembling could also be due to the anxiety caused by a dog not feeling well. It's ok to make sure he is comfortable and put a blanket over him and let him sit by you - but be careful about the consoling. If anxiety is combined with the not feeling so well, sometimes a consoling attitude by the owner can actually make the anxiety worse ("It's okay" "poor dog" said in a consoling manner can backfire and work to increase instead of decrease the anxiety) - It's better to act calm and confident instead of consoling. A massage method called the Tellington Touch (TTouch) may be something that you could do to calm your dog. Check out www.trafalgarsquarebooks.com.
Good luck. I hope I helped you think about what may be causing your dog to tremble and pant at night. Also, keep working with your vet on the problem and also seek information from the veterinarians on Just Answer in case they have a different slant on the problem.