I am very pleased that Jax' appointment went so well. As you said, having his nails trimmed for the first time, especially with all of the other poking and prodding, probably made him a bit anxious. I'm sure, with time, that too will pass. Popping into the Vet with Jax, even without an appointment to just say hello is a great way to continue his feeling of goodwill with the Vet's office.
That's a shame about the play date with Shelby. Perhaps the timing of his first play date wasn't the best. I was thinking that as I read your post. Obviously it didn't occur to me prior, or I might have mentioned it. Considering how much he went through at the Vet, and then went on to a play date with a much bigger, more rambunctious dog, he surely must have been tired out and probably felt more vulnerable than he would on a usual day. I'll bet in two more months, when Jax is a little older and bigger and hasn't just come from the Vet's, he may well enjoy a good romp with Shelby. Never say never.
I bet Jax is still exhausted today! Enjoy the slower pace as it will be over tomorrow for sure. Thanks for the update.