Why did my friend complain about me to our mutual friend right in front of me?

I have this friend at work, and the other day, she started complaining about me to our mutual friend when I was well within earshot. She claimed I never had to send our shift report at the end of our work shift, which was not true because I sent it the day it was just the two of us in the office. But she was complaining about me never having to send the report out and how they should "make" me do it at least once. I did not realize she had any kind of issue with me, so I looked at her stunned. When we made eye contact, she kind of just smirked and continued complaining about me. I also noticed she started sighing at anything I said even if it was not directed to her. I don't know what I was particularly saying wrong, so I just stopped talking the rest of the day, unless someone spoke to me first. She seemed fine after that and just continued speaking to everyone else in the office like usual. But when we went to clock out, it was just the two of us walking together and she suddenly started speaking to me like nothing happened. Why?

Why did my friend complain about me to our mutual friend right in front of me?
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