Hi, I just brought a 4 month old kitten. We had him for about a month before having to drop him off at the shelter vet. He's home now and for the first couple of days he was superrr affectionate and his usual self but once we started introducing him again to my older cat he has been growling and not as affectionate as usual. They aren't friends yet but they don't fight a lot. What could it be? Any advice?