Loving family suddenly turned to vicious fighters

I have three indoor cats. I received Luna, the female, when she was 8 weeks old and now she is 2yrs old and has always been a little more aggressive than the other two cats. I got the other 2, brothers, smokey and BAMF when they were 4 weeks old because they were living in a bad situation. I have had them for more than a year now. All three were fixed when they were about 6 months old. Luna took to the boys as if they were her babies and all three have always gotten along together very well playing together and often cuddling together.
About two days ago things suddenly changed. I came home from school and Luna was hiding. When I found her she seemed aggitated and upset for some reason, but she allowed me to pet her and move her into the kitchen for dinner. As soon as she saw the boys her ears went back and she began to gowl and hiss. Smokey, the larger of the two, attacked Luna and they were seriously fighting until I made a loud noise to scare them apart. I fed them separately and thoughout the night everytime Luna saw either of the boys she began to growl and hiss and whichever boy she was upset with would attack her.
The next day I took Luna to my house, leaving the boys at my boyfriend's house. While separated they all acted normally, after a night of separation we reunited the group and Luna began to growl the moment she saw the boys and they fought like I have never seen before. I don't understand what brought on this strange behavior between a group that has lived together and loved each other for over a year. This has been going on for about 4 days now, and I am at a loss of what to do so that they will get along like they used to.

This happened with my Cats too. There was a Pit Bull that tried to attack my Cats through the Window and a Orange cat kept going after my Cats through the Window too. One day I came home and both their Pets were causing trouble and the people didnt even stop them. I go inside my Apartment and hear screaming. I go in the other Room and Stormy is hanging by the cord to the Blinds screaming. She was hanging by her back Paw and it was almost broken. Coco and Meeko were having a big fight and Cocos Paw was hurt too. After that Coco would attack in of the Cats if she saw them She was very Mad. She went after Yoshi my only Male too. It went on for 6 months then one day she stopped being mad at them She still hisses if she goes in that room. Stormy was Cocos girl and they never were friends again after that. Stormy died last Dec. Stormy refused to forgive Coco because Coco had attacked her. Have your Cats seen any strange Cats outside? Coco and Meeko are best friends again. She is also very good to oreo my Kittens and Sasha. This was all because of my Neighbors Pets. My Sister has 2 Cats and they are fighting too. A strange Cat came to her window and then they fight for no reason. Its been going on for 3 weeks. I wonder if that is your problem.

There are a number of possible explanations, but the first thing that came to my mind was that perhaps Luna is sick, even if she's not showing any outward signs of it (cats are experts at hiding illnesses).

Cats rely greatly on their refined sense of smell to identify other cats. When one cat is sick, he/she smells differently to the other cats. This unusual and confusing smell bothers the other cats, and they often react by behaving in a hostile manner, as is the case here.

But then again, you said that on at least two occasions, Luna is the one who made the "first move" so to speak by growling (seemingly unprovoked) at the boys before they attacked her. This wouldn't seem to fit in with the "Luna is ill" theory. Maybe she just got into a really bad fight with one or both of them and it's going to take some time for a return to normalcy. Especially since they all always got along well --- they're not used to this new (and hopefully temporary) dynamic.

I had a similar situation a few years ago with littermates that suddenly started battling with each other. Their initial fight was so bad I had to go through the process of reintroduction.

I would definitely recommend separating Luna from the boys when you're not around. She needs to feel safe, and the time alone in her own space might help.

Another thing I learned: unless they are engaged in a fight, if there is tension, try to let them work it out on their own. Try to be as quiet and still as possible and observe to see who is the aggressor, etc. That works for me now, but your situation may be different.

Good luck! I feel your pain!

Thanks for the advice everyone. We went to the vet yesterday for a slightly overdue checkup and to ask what they thought and he said maybe the scents of cats marking their territory outside are upsetting her, and suggested the feliway pheromone spary to try to help her calm down. I don't know if anyone really knows whats going on with Luna, but I think I agree that she probably got into a bad fight with one or both of the boys and now she is really mad at them. Its interesting though because she definitely seems far more agressive at my boyfriend's house. When they are together at my house she must feel more comfortable or maybe the vet is right about the outdoor smells. She still growls at them there, but at least we don't have such bad fights. I don't know I guess we will hope that they work it out otherwise we will be back at the vet if things dont improve over the next couple weeks.

I am going through the same thing. Someone told me it is mating season and that Toms are likely spraying a lot.