WTF was up with my friend?

The other day I met up with a friend and she acted kind of weird. We used to work together and have known each other for about three years. We met up after I went to work (I opened the store at 6am and worked until 4pm, so wasn’t feeling too good) and she went to school. We walked around and decided to do a little shopping.

During our visit she seemed spaced out, unenthusiastic, and distracted. She is usually very warm, friendly, and outgoing, but I had to initiate most of the conversation and she kept wandering away. . She started a conversation with a guy in the store because he was wearing a t-shirt with a band she liked, and she seemed way more excited to talk to this guy than to talk to me. Later we ran into a girl she knew- she turned to me and said, “hold this” and handed me a shoebox she was holding so she could go into her bag and get something. Then she had a brief conversation with her friend and didn’t introduce me. I had to go to dinner with my parents, so I left a few minutes later.

This is the first time my friend has acted this way toward me. To be honest, she usually makes more effort toward keeping in touch than I do- she was the one who initiated this get together. I’m pretty irritated with her behavior, but this is the only time it’s happened. What is a possible explanation?