A few nights ago, we woke up in the middle of the night to hear our two female cats fighting. Normally they fight as play, but this sounded serious. There was a lot of hissing and growling at each other. Then last night they were hissing at each other. They've been together for almost 3 years now, and this is the first instance of them fighting. Any thoughts. There have been no changes to the house, other than we've temporarily moved the ferrets downstairs.