Siamese cat suddenly started being aggressive towards our other cat?

[squittensmum]

Anyone got a clue whats going on here?  A week ago, Star (half Siamese female ,  spayed, 9 months old) had a bit of a set too in the garden with a cat who lives two doors away (Billy, tabby, 5 years old spayed female), I heard it inside so went out and my other cat Squitten (Siamese spayed female 8 years old)  jumped up and  also went outside to see what was going on. Billy ran off at the sight of her (Billy & squitten don't get along, they have fought before) and star was OK (billys tail went "Squirrel" but stars wasn't)

 About 5 minuets later, squitten is beating star up! shes never done that before, we've had star since she was tiny and squitten helped raise her, taught her to hunt, shared her garden & food  with her, their a team.

 Or so I thought, we've had a week of hell now, every time they see each other star starts growling and backing into a corner and squitten goes for her, we're having to keep them separated but we live in a small bungalow, its causing a lot of trouble, we also have a third cat whos now scared stiff of them both and just hides.   Star is my partners cat and Squitten is mine so you can imagine the stress its putting on our relationship, she also has a history of bad behavior (piddling on beds and general spitefulness) but the last couple of years Squittens been wonderful so I don't understand why her behavior has suddenly changed. 

 We've had feliway diffusers in use since we got Star.

Anyone got any ideas? am kinda at my wits end & terrified i'm going to get asked to re home my beloved Squitten.

 

[yayi]

During the confrontation with Billy, some if his "scent" may have transferred to Star. This could have triggered Squitten's aggressive behavior towards her. Star was stunned by the attack and now she is wary of Squitten who then gets upset probably taking it like Star is challenging her. 

You may have to "reintroduce" them again. Concentrate on scent exchanges.http://www.thecatsite.com/a/introducing-cats-to-cats#comment-list You also mentioned Squitten had a history of "general spitefulness". How did you solve it then? 

 

[squittensmum]

Will try the old scent towel trick on them, Billys a spayed female so wasn't sure if that made a difference. Squitten has been known to really nip the tender spots under other cats legs when she thinks no ones looking as well as other small nasty things, never really solved it to be honest, she stopped piddling on our bed when I started letting her into the garden when she was 3 (we lived in a flat before that) but shes slowed down a lot as shes got older and until last week was wonderful with Star. We've had a lot of ups and downs this last year so she has had to accept a few changes but in this house-we do not attack each other, whatever species we are and she needs to learn the real fast cus my partner will not put up with his cat being attacked every time they meet.