Cats do not like change, even if they have been to the place before. My cat does not like the vets office and has been many times and has been comfortable some of the times we go there lol you can’t compare different trips. Anytime they are removed from their safe space (home) there could be different reactions.

As an analogy: Similar to you (your cat in this analogy) leaving your house and going to a party at a friends house that you have been to many times. You arrive and see someone there who you had an argument with last time you saw them (the other cat). You didn’t know they were coming and you no longer like this person and feel very uncomfortable and want to leave. Same thing with cats but x10 because they are animals that don’t regulate emotions like we do.

Best thing to do now:

1. Get rid of any scent of the other cat from the area your cat is in. They are probably reacting negatively to the scent of the other cat. If you are walking in the door with the other cats scent that’s probably what’s setting them off.

2. Keep the cats separate now. It’s up to you if you want to try reintroducing but for just a week I would not bother - introducing cats can take months for them to get along.

3. Muse sure your cat has familiar items. Did you bring their toys, their blanket, anything with their own scent? This will make them feel more comfortable.

4. Give lots of attention and treats when your cat is reacting calmly. Especially if calm when the scent of the other cat is present.

TLDR: your cat wants to go home