radical behavior change

Question
We adopted two female siamese kittens from the same litter.After about the third heat and 7 months we had them spayed at the same time last week.These lovable dosile submissive (toward the other two older females)kitties have turned into two cats from hell.Aggressive mean attacking cats.This has happened almost overnight.They also cower and have an almost gutteral cry.
 Help!!

Answer
The first thing that comes to mind since they were so recently spayed and they are both acting like that and especially cowering and making that sound is the possibility of some sort of infection. I would first take them or at least call the vet and see if they can't get looked at. The other possibility that I think about is since they went through  three heats, they may wonder why all of a sudden they feel strange and not know why and so they act out on the other cats. If there's no sign of infection, I would try to comfort them when you can and see if they start coping with the change.In the mean time, go ahead and have their food/water/litter box separate from the other cats. And don't be afraid to use a water bottle if they too out of control. I hope this helps. Keep me posted on how things are going.