Cat behaviourists anyone? Need some advice please. 

[Lilypad34]

Kitten is almost 19 weeks, resident cat almost 16.

Older cat took to the kitten after about a month, loved her groomed her they both played together suddenly over the last few days he seems to hate her.

He doesn’t chase her doesn’t corner her or behave aggressively but if she comes anywhere near him he growls a really deep growl and will continue to grumble even after she’s gone away.

When she does decide it’s playtime he growls and will smack her and hiss but no claws out doesn’t bite her and she doesn’t seem to care.

She’s due to be spayed next week, the vet thinks it’s due to her hormones her scent has changed. It’s like he doesn’t know her. Can anyone give me any advice?

We’ve gone back to separating them and starting the reintroduction again. Thank you

My first thought would be is your older cat feeling unwell/in pain. They most likely have arthritis at that age (90% of elderly cats do), are they on pain meds? Maybe a sore tooth, something like that can cause a change in behaviour.
A pain trial is the easiest way to see if their behaviour changes.

That was my thought he’s going for a check up on Monday.