One of my 2 cats started acting aggressive towards the other after 6 years of no problems

Hey! So I should note that my cats are primarily outside but used to come to my room a lot through the window during winter. They’re both females: Ruby and Poppy. Poppy is a month or two older and acts a lot dumber, while Ruby is mischievous and often too smart for her own good. They used to wait at the window together, come into the room together, and even sleep next to each other. They have never fought before, not even hissed or growled at each other.

About a month ago, Poppy got attacked by some animal and had a wound on her head near her eye that got infected. We had to take her to the vet multiple times, and she had to wear a cone for about a week. Ever since then, Ruby has started absolutely hating her. They can eat together and be outside together, but whenever they’re in a closed space like my room, Ruby growls and hisses, her eyes are completely black, and she curls into a ball in a corner.

I don’t know how to fix this. I can’t take them both inside when it’s cold because Ruby will react badly. Poppy doesn’t seem to care that Ruby dislikes her being around; she keeps doing her usual activities. Would it help if I took only Ruby inside first to get her used to it, then added Poppy? Or should I wash everything that smells of Poppy? Any advice would be appreciated!

It’s not uncommon for cats to change their behavior after a traumatic experience. Ruby may be reacting to Poppy’s altered scent due to the stress and trauma of her injury. Try reintroducing them slowly. Start by allowing Ruby to have quiet time in your room without Poppy, then gradually introduce Poppy while keeping things calm.

You could also try scent swapping. Take a blanket or toy that smells like Poppy and let Ruby sniff it. This may help her get used to Poppy’s scent again without the stress of direct contact.

Consider using calming pheromones like Feliway in the room to help both cats feel more relaxed. It might ease the tension and help Ruby feel more comfortable.

Make sure to give Ruby plenty of positive reinforcement when she’s calm around Poppy. Treats and praise can help her associate Poppy’s presence with good things.