Sudden behaviour changes in puppy

I have a parson russell pup who had just turned 1, had her since 8 weeks. She lives with our older whippet, attended puppy training classes and has generally always been a sweet, happy pup who loves people and other dogs. In the past month or so she has really changed and has started growling and barking at some people and dogs. It seems to be mostly bigger dogs, but it seems completely random which people she growls at and which she just ignores. She’s at her worst if we’re on a walk and there aren’t people around and then suddenly someone appears and she spots them. I really don’t know what has triggered this and haven’t been letting her off lead as I don’t want people thinking she’s aggressive and a risk to them or their dogs. I’ve tried to reward her when quiet and mark it with a command which sometimes works but she doesn’t listen once she gets going. She is getting spayed next week which I’m hoping will calm her down a bit, but does anyone have any tips or had the same experience with their dogs? She’s otherwise healthy and well behaved. TIA