Dog is different after a vet visit

I've had a Chihuahua dachshund mix for about a year now. He's 3, a bit reactive and anxious, and I presume I'm his third owner.

He has always been a Velcro dog. Until now. I took him to the vets for his annual shots. And suddenly he's not really Velcro anymore. Which is kinda a good thing but still a weird change.

I understand that rabies shots can make a dog feel sick.

For instance before if I got up to go to the bathroom or do something, he'd follow me no matter what. Now if I get up he just stays there until I wake him up or get his attention.

It's like he's suddenly comfortable inside the house. Outside he's a non stop barker typically. At the vet he just paces constantly.

Or maybe I'm overthinking it and the dog has finally learned my schedule and the household?

He was treated for heartworms last year so I don't know if that makes him act like an older dog?