I have a 5 month old female kitten (Stella), and I just brought home a 3 month old male (Oscar) a little over a week ago. when I first brought Oscar home, I let Stella meet him when he was still in his carrier. She was fine, just curious, and so was he. I kept him in his own room for a few days and then decided to let him roam the house. They sleep on the bed together and have no problem eating next to each other, but for some reason, any time that Stella tries to play with Oscar, he starts hissing and growling. Keep in mind that this is the new kitten that is acting this way towards the resident kitten, rather than the other way around. Any idea why this might be? Again they are comfortable in the same room, sitting next to each other, etc.