william's suddenly afraid!! U/D please HELP

I usually take William and Charlotte to the vet or rabbit rescue for their nail trims (I still haven't managed to do it properly myself). They were all booked today, so I got an appointment at a walk-in animal clinic. They've done William's nails once before and did a good job. We had to wait quite a while with lots of animals before us. Charlotte didn't seem to mind the wait and watched everyone curiously from the carrier, but William sat tightly in thr back. When we got home, I just opened the carrier up so they could play in the house. I think I should have put them into their playpen instead, because William was so upset and afraid!

He went straight to his "pouting" corner and stomped once or twice. He seemed fine though, as that's his usual reaction to coming home from the vet's. Charlotte was fine - playing with the new wicker ball they got as a present for nail clipping
I went on with some cleaning and William suddenly ran flying up the stairs - he went straight into the bathroom to hide behind the toilet. When I came up he ran downstairs again to sit behind the couch. He stayed there for a couple hours. I finally saw him out on the floor cuddled up to Charlotte as she groomed him. But it took a fw more hours untill I could get him to go into his playpen and shut the door. I'm hoping that after a night together in their area, he'll be back to normal tomorrow morning. But he was so frightened. I was afraid he'd lost trust in me for some reason.

I just woke up at 6am to William thumping loudly... He looks so alarmed. As if he hears or smells something that's scared him. Charlotte is fine - just wanted her breakfast and a few cuddles. But William won't stop freaking out. I don't know what to do!!!!

It sounds like the vet trip has really freaked him out.
Could he see a dog or something like that?

William was in his carrier with me. Then the vet's assistant took him back for his nail trim.
There were a few dogs in the waiting room and one little pomeranian that barkeda lot, so he would have heard that. I don't think he ever saw them though.

Hmmm, is he usually quite a timid bun?
My first guess would be that he has gone into his natural instint after experiencing what he considered to be a predator.

He is somewhat timid.

How should I help him relax? Normally on weekends, they get free roam of the house, but instead I'm going to leave them both in the pen. Will he calm down over time as he see's he's safe and at home?

I wish I'd never tahken him to get a nail trim!!!!

I'd suggest confining him to a small familiar space with Charlotte until he has calmed down, and then giving him a few treats and attention.

Ok, I'll see how that works. It's just 6:30 in the morning here so hopefully a long day in their area and a few treats will help reassure him.
Thanks.

William calmed down a bit when I came to sit nearby with my coffee and the TV on. He had some of the greens I gave him and a sliver of banana. He's still a bit nervous, but seems like he'll calm back down.
Right now he's stretched out beside Charlotte resting in their sleeping area

It's just 8, so maybe after lunch I'll let him play in the living room and see if he's ok with that.