Let me give a little backstory. Two years ago we got sibling sister cats. When they were about 10 months old, we got another kitten. The older two were OK. They didn’t really like her but they weren’t mean. Fast forward to 10 months later. We found out that another kitten was in desperate need of a home. She just got fixed yesterday, and we brought her home today. She never had any interaction with the other three cats, as she went right into a room to recuperate. As soon as I put her in the room, I close the door. One of the original catsImmediately started hissing at me. I don’t know what happened. She never had contact with the kitten, but instantly became hostile towards me and the other original cats. she was my little shadow for as long as I can remember. I can’t even pet her, or look at her for that matter. I don’t know what happened that made her soo angry. We intend to go through the entire introduction process as we did with the other kitten we brought home. I just don’t know what happened that OG cat is angry with me. I’m worried that whatever is going on with her won’t be undone. I’ve never heard of this type of behavior and all I’m finding online is aggression towards new kitten - not the own. I literally can’t go near my cat without her growling, pupils huge, and hissing repeatedly. Any suggestions? Ideas?