My cat(s) have become aggressive after spaying. What do I do?

I have an 80 lb just neutered German shepherd and at the same time I spayed a Siamese and a calico a week ago today. About a month ago I acquired a 14 year old 10-15 lb dog (Yorkie/Cairn terrier) The Siamese has tried to fight the little dog the last three nights and then either the calico joins or gets beat on next. These two are so lovey dovey during the day it's almost gross. This is completely out of character for the cats and the poor dog is terrified. Why and what do I do?