Why is my dog aggressive towards her housemate after being spayed?

I am very sorry to hear that you are experiencing these issues with Hazel and Daisy. I believe a couple of possibilities could be occurring. Though only behaviors that will be affected by spaying are those that are under the influence of female hormones, it could be possible that they are both still undergoing hormonal changes and so they need time to adjust. The hormone changes can take up to a few months to stabilator.

When everything is stablized, a dog's temperament, training, and personality, are related to genetics and upbringing, not hormones, so should not be affected. Neutering does not “calm” an excitable dog, and unless a dog is overfed or underexercised, there is no reason for it gain weight or become inactive.

The undesired traits which are related to hormones such as sexual behaviours, aggression, and urine marking will change over the course of a few days to a few months.

It is also possible that Hazel is possibly not healing well and still experiencing some pain or discomfort. I would try gently palpating her abdomen to see if you get any flinching nor pain reactions. A check-up with the vet to confirm she is not in pain would also be a good idea. Pain can be expressed through changes in mood, which are often outwardly viewed as aggressive behaviour.

I hope that is helpful information and I wish you the best of luck with Hazel and Daisy.