We received two beautiful 10 week old kittens on Friday from the Cats Protection, litter mates with no medical issues. One of them has relaxed very quickly, plays with us let's us stroke her, eats well, uses her litter tray and is totally fine. The other one seems to have gone the other way, she is very quiet, sleeps a lot, hides away, eats but not much and isn't interested in food, I've seen her use the litter tray but not since last night. She doesn't play as much as her sister, and has growled at her on some occasions when she tries to play with her. She sometimes goes to a corner of the room and meows as though she's calling for mum.
I've texted the cats protection rehomer for advice but she hasn't replied yet and I'm a little worried. Does she just need more time to settle or should I be looking to take her to a vet? What are we doing wrong?