I have two cats - both are 7 years old and we've had them since they were kittens. They have always tolerated each other and only ever play fight. The one - called Kitten - has always been the friendlier of the two, always so playful and happy. Then a few months ago, we brought a lost kitten into the house and Kitten went absolutely crazy, she screamed and hissed and wouldn't let anyone near her. She calmed down the next day but then a week ago she saw another cat through the window and she resorted to the exact same behaviour...three days later, she calmed down. Now yesterday, I was upstairs in my bedroom playing with her and she was purring, licking me etc and then she saw that my other cat had come upstairs and she hissed and when I tried to touch her she went mad again. Now she's hiding anywhere she can find and whenever she sees me or senses i'm in the room she screams and hisses and makes a violent swiping motion with her paws. It's like she really hates me - all I have to do is be in the same room as her and she just screams at me like she's scared of me, but I don't know what I've done. This has happened three times now in the past 5 months and the last two times have happened in the past week...she's still not calmed down at the moment. I don't know what to do, I'm so upset...I keep thinking if she remains like this or keeps having these mood swings frequently we're going to have to give her away. Can anyone tell me why she is behaving like this and if there is anything that can be done? Thank you.