Welcome to Pet Talk! Karen may be onto something here. Cats can smell it when another cat is ill or becoming ill and, even if they've been good buddies all along, the stronger cat may bully the weaker, sick cat. My meezer Coco Puff and my calico Puddy were pretty good friends until she started to go downhill w/chronic renal failure and her breath smelled funny. Then Coco Puff picked on her after that. It's a good idea to get your kitties to your vet and see what s/he has to say. A checkup for both of them may answer your question. Good luck and I do hope that things calm down for you. I'm looking forward to a good report.