Cats fighting after being spayed/neutered

I brought my cats home from the vet today. I had them spayed and neutered yesterday. They are almost 6 months old, a male and female siblings, very close. But, for some reason, now they are fighting. I had to separate them and put one of them in my room so they wont hurt each other.  Why are they fighting? How long should I keep them separated?

They're in pain and feel weird from the anesthetic. If you watch Youtube videos you can see that people do/say some pretty strange stuff while recovering from surgery :lol3:. So just keep them separated until they're feeling better. They might be OK tomorrow, but if not, keep them separated another day.

I hope so, keeping them separate isn't easy since they use the same litter box. Thankfully they're napping now! Thank you for the advice