Fixing a dog leads to chemical changes in the dog. Especially in females were various hormone levels will drop -- even a female dog out of heat has higher hormones of the fertility kind when not fixed than when fixed. Dogs rely heavily - much more than humans - upon scent. Any changes in the scent of a dog makes them either 'sick' in which case pack mates will either react more kindly to them or become more aggressive depending on how the illness affects the pack. Or they will treat the dog like a 'new' interloper in the pack. Different scents means different dog. Fertility can be sensed by other dogs. Part of the pecking order of the pack depends on the fertility of the females. Those who are more fertile get a higher station in the pack and get more attention. Those who are less fertile are stationed lower. If a dog's fertility suddenly changes the pecking order is challenged. It should resolve itself over time - how much time depends on the age of the fixed dog - the older she is the more effect the change will have, and the personality of the aggressive dog. Wait about a month, if things do not settle in that time its time to seek a dog therapist - most dog trainers have some knowledge of dog psychology thus can be considered therapists, or the vet. If the aggressive behavior increases, then separate the dogs and call the vet. S/he will ask many questions and may want to see the aggressive dog. Aggressive behavior can be expected when the other gets fixed, however aggressive behavior can be a symptom of other illnesses and disorders which could be taking place in the aggressive dog just as a coincidence of timing having nothing to do with the other being fixed.