I have three cats. Since March of this year, we had rescued a cat from outside and she got along with the two cats we already had just fine. She is best friends with one of them. She never really had an issue with the other one since she pretty much stayed in our computer room out of way and when they were near each other, no issues.

Since yesterday morning the newest cat we rescued has been growling, hissing, and attacking our shy cat who 'stays out of the way'. She is doing this with a severely arched back with hair standing straight up on her back and poofed out tail. It is so scary. I worry about the cat she is after.

I am so nervous that when we leave for work today that they will 'fight'. She plays well with the other cat whom is her best friend, so their fights are playing. These are true fights. Right now I have them separated since I am worried. What can I do? Any ideas/suggestions are appreciated. Thank you!

Try Feliway and catnip.Both of these items work as a calming device,and many female cats develop attitudes when they mature.Hopefully it will pass,but I feed all my cats separately because of conflicts at feeding time.The one cat that is upset is trying to show dominance and prove herself as the queenin your house.

There could be an illness with the shy cat, so you might want to get a check up. Some cats
Can see the weakness in another cat, and start acting aggressive.

Is your shy cat a female also, and was the rescue over a year when you took her in?
Often females don't get along well with each other in close environments, and if you
Rescue was under a year, she could be trying to let you shy girl know who's boss.

You may just need to keep them apart for a week, and then see what happens when you
Try to integrate them back together. Sometimes that works. If you end up not being able to pinpoint the problem, you may need to segregate them forever. I ended up having to do that with one of my cats. She lives upstairs now. Good Luck!