My 9 y/o lab suddenly started having trembling of his body about 8hr ago. He was fine this morning, ate his food and supervised outside while he pooped. We left for an hour and came back to him:

-Trembling body, seems to originate in butt or back legs
-Tail tucked
-Following me around much closer than usual, seems very insecure
-Sitting in non-typical places around the house
-Some intermittent heavy panting, but that's normal for him
-Doesn't seem to want to drink water

He is not
-Vomiting/Diarrea
-Whimpering
-Having any mobility trouble. Ran down the stairs, jumped on the couch, etc

He just ate his dinner with no ill effects yet. Any ideas?

Is he in pain? sounds like my GSP when she has pancreatitis except she was in pain too. It can be deadly.

He isn't vocalizing any if he is in pain. He was hesitant to go down the stairs just now, but he pooped and is now drinking water like a camel.

It does cause dehydration. And those stairs might be causing some abdominal pain. I don't think he would have much of an appetite tho and said he ate..take him for blood labs either way.

He is in pain somewhere IMO. You say he is tucking his tail... If you have had him water training or swimming a lot see if the base of his tail is sensitive to pain. Could be a bad case of Limber tail. If not something obvious like that, then a trip to the vet would be in order if it is still going on.

Asking a bunch of folks who can't remember if they stayed at a Holiday Inn about your dogs health is pretty sketchy at best. Making your own diagnosis can be real problematic. If you feel your dog has a problem let a vet look at him.

Vet was closed yesterday.

Anyway he's doing much better this morning. Almost back to his old self.

Just curious, has he had any doses of NexGard or Bravecto? I mean anytime, last year 6 months ago or yesterday?

No sir, just his heartguard the 1st of every month

The reason I ask is I had a dog that we gave Nexgard and it killed it DEAD. After looking into it we thought of a dog we had a few years ago presenting similar symptoms. we determined it was the same meds and reason our first dog died. After some research, simple google, we found more people with your dogs symptoms and the list of meds were Nexgard, Bravecto and Heartgard, with Heartgard being fewer. Just something to look into. When you start researching you will see identical symptoms.. Our last pup that died was 8 and healthy, had just had recent check up they gave us a dose of the Nexgard and she died 4 days later... It seems these are basically poison that is put in our dogs systems and its a low dose that affects the target organism but our dogs body is supposed to deal with it. Seems over time some peoples animals get a toxic build up and some die on the first dose.. ours was its second dose... our first dog had a couple years then started the sympotms you describe.. back legs weak one time he fell while walking.. panting nervousness.... These meds affect our dogs nervous system.. read this and take from it what you will. our dog presented the panting and extreme nervousness for a while.. then second dose died. Some dogs have the mdr1 mutation which when they have this these meds affect them in a negative way it seems.... there is way more info on the web than I can recite here... I chose to go all natural for our new pup. Hope he gets better and no ill affects remain..
The link below has some info.. and I beleive I saw some on Heartgard
https://www.facebook.com/groups/704330073037200/

Glad to hear he is doing better. Things like that are scary.