I just got both of my kitties spayed last Monday, one kitty is doing great. The other kitty attacked the people at the clinic, she attacked her sister. When we got her home we had to separate the two of them. Now every time she sees other kitty she growls and hisses... They used to play all the time.
I'm not sure if its just a side effect, trauma from going away having belly cut open and insides ripped out, or if she's just turned to not like other kitty. She's sweet to her people though. I had her in my bed last night, she slept just fine.