I know how you feel, after I brought Joey home from her extended stay at the vet, the Bob would only hiss at her, and they had been getting along so well.
They seem to forget so quickly, don't they? And all the different scents on them (for example after a vet visit), or a change in their behavior (after sickness, or under influence of meds) doesn't make it easier...

bisi.cat, actually I was wondering about that, too! Lily isn't spayed yet, but I have made an appointment for Tuesday. I hear from all sides "she's too young", "she's too small" and "It's cruel", but she's 6 and a half months now, and she IS slowly getting into heat, and of course I don't want her to feel too superior - well, she does anyway!

Kirsten