Bit of background. We got two rescue cats just over a year ago. A 6 year old male called Leo and a 4 month old female called Maggie. Both had been desexed. We brought them home two weeks apart, and kept them separated but they kept trying to get to each other. Under supervision we let them meet, they showed no hostility and have been close ever since. They sleep curled up together, groom each other, just generally get along really well.
We have a ten year old golden retriever that both cats have got along with from the beginning. Basically all the animals are friendly and affectionate with each other.
The cats are predominantly indoor cats, our neighbour has nasty dogs that would kill a cat if they went over the fence. The cats are allowed out when we supervise them.
So to the problem, this morning I was woken up by yowling and hissing to find both cats under the kitchen table. They weren't fighting but were both being very aggressive. The dog got between and they both ran off.
Since then they have both been slinking around and avoiding each other. When they do spot each other there is more hissing and growling. It appears to be Maggie, the younger female cat, who is attacking Leo.
Nothing in their life has changed recently, so I can't think of a reason that either of them would be stressed. They both recently had check ups and are healthy. Both cats and dog are wormed and flea treated monthly.
Does anyone have any ideas what could be the issue? At the moment Leo is hiding in the bathroom and Maggie is slinking around the rest of the house. I just want them to be friends again!