Before our honeymoon, our cats Mary and Amy were happy friendly sisters. Now, they are hissing and arching their backs whenever they're near each other. It's very out of character and sad to watch.
According to the internet (especially the comments on that post), formerly friendly cats can end up suddenly hissy at each other for several reasons, all of which apply to Mary and Amy, unfortunately:
- a new environment (eg, moving to a new house)
- one cat going to the vet, but not the other, so that one has "funny smells" the other doesn't recognize
- redirected aggression (eg, seeing a strange cat out the window that they can't directly fight, so the pent up aggression gets taken out on the cat they can get to)
My sister took care of our cats in her Sacramento apartment, and now they're back home after 5 weeks away. Amy (uncharacteristically) starting peeing on the carpet instead of the litter box, so she took Amy (but not Mary) to the vet. And my sister had commented that Mary was constantly looking out the window at the neighborhood cat. So that's like the trifecta of Bad Times, cat-wise.
I just hope that they work out their issues soon; it's really sad to hear them hiss at each other all the time now. :(