She could be in heat if the vet didn't remove everything, but she could also have an infection. Worth having her checked by the vet.

I took her to the Vet and she said that she is fine and healthy, no false heat. Time for her shots anyway. She also said that the male cat probably is just curious and playful since he is younger. And that he probably likes her smell! But she doesn't like him back!

They have a history of fighting, when he first arrived as a stray living outside,they fought often when she went outside. I had him neutered hoping he would settle down and he stayed outside for a long time but I finally started letting him in.

I think she probably expects a fight when he is messing with her in any way. So she throws the first punch!

Cats, like dogs, sniff each other's butts - in or out of heat.

And I have the same situation - neutered female and male, and he wants to be friends and she's distrustful of him.