Called me her friend and I don’t know what to do?

Anonymous
My girlfriend of 2 years called me during work and we were chatting about our day and such. I overheard one of her coworkers asked her “So where’s your best friend at today?”

She told him that she doesn’t have a best friend at all. I asked her about who is this “best friend”? She said that she doesn’t have a best friend and claims that her coworker were just messing around. I felt a bit unsure about it and kept it pushing.

We continued talking on the phone and another coworker asked her who she was on the phone with. This shook me because she told him “With a friend.”

I spoke up and asked “who she’s referring as a friend?”

She told me that he doesn’t need to know her business & I already know what I am to her. After that, I hung up on her and just sat down in my room thinking. I don’t know what to do at all. I feel stuck.
Called me her friend and I don’t know what to do?
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