Cat acting strange after other cat goes to vet and back

I will try to make this short, I have two cats, a male and a female (both fixed, been together since day one), they are 9 years old. I took the female to the vet for shot's to help with her dry skin, she got an antibiotic and a steroid shot, we were gone less than an hour and a half. It will be a week tomorrow and since then my male cat hisses at me and the female cat, growls at the sight of either of us, has made a new home on top of my kitchen cupboards (has to go from counter to fridge to top of cupboards), I'm assuming he's eating and can only assume he's using the litter box (haven't found any spots to prove otherwise).

So my questions is, what do I do about him? He's never acted like this, before a week ago he was the sweetest, most loving, silly cat every, I called him a momma's boy, he could hear me get a blanket and lay on the couch from a mile away and would come running, him and his sister used to get along great, now he hisses and growls at both of us and wont come down from the ceiling (basically) unless I'm gone or we are sleeping.

Answer from Kate
Hi
this sounds like a classic case of truama aggression.

this is a common issue which is caused when something happens which frightens one cat and they blame that fear onto another cat or person. In this case the smell of your cat after she went to the vets may have been the trigger.

his behavior is caused by fear and so what needs to happen is for his confidence to be re built between your two cats.

the process best used for this situation does take a little time and effort but really can help. basically you need to take your cats through the reintroduction process described here

https://www.our-happy-cat.com/introducing-cats.html

best wishes Kate

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