One of my two cats - let's call her Cat A- (they are both 3 year old girls, spayed, not sisters but had them both from kittens) has just come back from a four day stint at the vet and my other cat's - let's call her Cat B- behaviour has dramatically changed - she is usually very outgoing and affectionate but she has been hiding under the bed and sofa, and if Cat A gets too close she hisses at her (this is completely out of character). I also don't think she's used the litter tray although I don't know for sure as we only have one (we used to have two but they insisted on using the same tray so we got rid of the second one). She's still eating and drinking.
Do I need to take her to the vet now too or is this just because she doesn't recognise Cat A and thinks we've brought in a strange cat?
We go on holiday Friday and I'm worried!! Our cat sitter will come in 2 x per day and is a former vet nurse so I know they're in good hands but even so.
Any thoughts appreciated.