My rehomed dog is becoming aggressive towards men

Recently (around 4 months ago) I rehomed a beautiful staffy/whippet mix. We found her on Pets4Home from a family of 6 (a mum, dad and four little boys) who could no longer care for her. She’s a great girl, in the home she’s so cuddly and loving, she’s great off the lead and is kind with other dogs.

There’s just one thing, she isn’t very friendly with new people but especially men. When we first got her, she didn’t show any signs of aggression, she was nervous and could seem stressed, but never barking or lunging.

She also recently went through a phantom pregnancy which I don’t think helped anything (we had to give her some hormones from the vet as it was continuing for longer then hoped and we wanted to get her spayed), but even now she’s over it, the aggression continues.

She doesn’t resource guard anything, she’s never been aggressive to us, she only really barks when someone knocks on the door. My parter is a male and she loves him.

The family we adopted her from really didn’t seem the type to mistreat her, and as mentioned it’s more of a recent thing. Myself and my partner never use physical punishment for bad behaviour and honestly rarely raise our voices.

I just don’t know what is making her so fearful and bark so aggressively at men. She’s never bit anyone but she can lunge and I’m worried one day she will bite someone.
Her bark is soooo scary and loud and I really want this resolved. some of our friends are afraid to see her because of this and it’s putting restrictions on our social lives.

Does anyone have experience of this? Possibly with reactivity related to phantom pregnancy?
We’re also having her spayed so would be helpful to understand if anyone has experienced how that affects behaviour in females.