Sudden inter-domestic cat aggression between usually friendly cats
Hello! Am new to this site and would like to ask for advice on our domesticated cats:
We moved house 3 months ago from apartment with terrace to small villa with walled garden. Our 2 cats (both previously adopted strays from 8 & 4 yrs ago) have adapted very well to new surroundings (kept inside for a while) and garden and up to yesterday both went outside during the day, sniffed around etc. Last night we got back home to find the older cat extremely stressed and aggressive towards the younger one - it's as if older one doesn't recognize her house companion anymore. Totally hissing and not accepting the other and very different weird behaviour. The younger one is more feral but we have lived with both accepting each other for the last 4 yrs together. Could it be to do with the fact that both have seen 'real' feral cats outside now as they sometimes come into our new garden and have picked up smells from them? As far as we know no feral cat has been inside our house since we moved in. But why the sudden inter-aggression? Older one also seems very lethargic and very hesitant - much more so than usual. Have booked vet's appointment for a check-up in case of anything serious. Any advice gratefully received, thanks!