11 month old puppy is suddenly reactive

TLDR: my people-loving puppy has suddenly become reactive

hi all, it’s me again. my girl is currently almost a year old now and we have come leaps and bounds with her training and up until recently i thought that she was very well-socialized. i was very cautious about who i brought around her when she was younger and what experiences i provided for her as i have seen what can come out of both under and over-socialized dogs. i thought that we were doing great, as my girl absolutely loved all people and would greet everyone the same no matter what environment we were meeting them in. she has always been good with other dogs as well- no resource guarding and friendly with other dogs, if anything she would just be a bit fearful initially.

it feels like in the span of a few weeks my puppy has done a bit of a 180 and has become more people reactive, but only with specific people. it started with one of my partner’s friends, who she had met when she was younger but a few months later decided that he was the most terrible person to exist and will lose her shit every time he comes over. i also just started bringing her to work with me again this past week, and she has decided that coworkers she has met previously are now scary and will bark and growl until they are out of the room. she will usually back off and become comfortable once the person in question has been in the area for a while and will then deem them as a safe person. however, this morning my girl would not back down from one specific male coworker and lunged at him, which she has never done and is raising alarm bells for me.

this is a long post, but i am just wondering how to proceed or what i can do to try to train the reactivity out of her before it gets worse or is cemented in her. i have heard that the ages of 9-12 months are formative in a dog’s life so it is important to try to work on unwanted behaviours, and i am just struggling a bit because this is a new thing for my puppy and i have also never owned a reactive dog. i am also wondering if living with my roommate’s people-reactive dog plays a role as well; some people have told me that dogs cannot teach each other reactivity but i have also been told that they can. i am just feeling a bit lost- i have been fortunate to have people that are completely okay and understanding with her fear and will work with her until she feels safe. but after what happened today i am becoming very worried and confused about where this is coming from and what i can do to correct it. i don’t want to ever punish her for having a reaction to a person but i am hoping for advice or insight from someone on how to proceed with working on this behaviour. thank you in advance!