What kind of behavior classifies a person as being passive aggressive?

I have a friend who seems like they do not want to share anything about themselves with me. I had asked her why has she been reserved lately, she answers ‘I have always been like this’. But that cannot be so, as I have known her for 6 years now. She later mentioned that she shares and talks more with her boyfriend. And I have noticed her behavior change progressively. I have been more upfront about the way I feel than she has. I do feel she is being a little passive. I am not quite sure. She has been rude in different ways lately, and she gives off a sense of prideful behavior around me as well. I just need to know what qualifies as passive aggressive?