Why is my female cat scared of my male cat?

[Erin]

This is kind of a long story-we have a male and female cat from the same liter. They grew up together, and we loved how much they hung out together and played, ate, slept together and everything. Even bathed each other. We suspected the male spraying because of outside cats running around our house, and we soon suspected the female to be pregnant. So, we got him neutered today, to potentially stop the spraying, etc. We brought him home, of course, he's sore and still drugged up, but he is him old self all day, a little more affectionate than usual. And he is drinking a lot of water, but made his way to the litter box and everything. Suddenly, we are noticing our female cat won't go near him--and when he tried to make contact with her, she hissed and jumped back, which she has NEVER done in her life. It was like she was scared of him, but he just wanted some contact with her. It actually brought me to tears because I was so saddened-they were the closest cats ever. Now, we thought it could be because she is pregnant and being defensive, but she has never EVER done that. Do you think it has something to do with him getting neutered? If so will she get used to it? I'm worried for both of them, and I just want them to get along. Thank you so much!

[KungFuGirl]

females cats fend off male cats if she has kittens, because a male cat will often kill the kittens.

however, a community of familiar female cats will group together at times and help each other.