Random encounter with friend?

I was walking home with my girlfriend to her home when I ran into a friend. She was with two people. One of two persons went to their car. They were the only people on that side of sidewalk near my girlfriend's home. It was a little past midnight on a weekday. I said, "Hi, whats up?" to the friend, but she ignored me and looked the other way. However, the other person (the other of the two) said hi and engaged with my girlfriend and I. My girlfriend is attractive and sweet. She consistently attracts attention from all sexes. She's amazing! The friend ignored me. My girlfriend ended the conversation with the friend. We walked away quickly, and my girlfriend stated that they did not like us. We are all in our 30s (5 people involved, my girlfriend, friend, two persons, and me). I was confused by the exchange, and upset. I previously had told the friend I liked her, but after months, she told me she had someone. So, I stopped pursuit, and met my girlfriend. I had talked to the friend a few months ago briefly via phone call for a business related matter, but we discussed some personal matters too. However, our exchanges are limited to nothing. I'm confused as to why she ignored me. I rarely have this issue with people. I tend to end things well with almost everyone, and if not we end on terms that we both mutually agree that we do not like one another, but have a mutual understanding as to why. Why did this "Friend" ignore me? I did not contact her after to tell her I had an issue with her ignoring me. I felt that might be absurd.

Random encounter with friend?
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