Exact same thing happened with my cats. They are brothers from the same litter and usually start crying if one is separated from the other. I got them neutered and when they came home they quickly separated from each other. Lots of hissing too. Which was weird because neither have ever hissed before. Kyuubi had a rough time as his wound was swollen and would attack Azrael every time he got near.
As time wore on and the vile stench of the vet wore off, these guys went back to being normal.