Hello guys, I need help in analyzing a situation that I had today.

I met this girl three weeks ago, and during those 3 weeks, I've only talked to her 3 times.

The first week: it was pretty normal, she initiated the conversation and I just followed along. We walked and sat in this one particular place. I got to know some details about her and then she left for class.

The second week: The same thing as the first week, but this time, she stayed even longer just to talk more. She said she is going to go eat lunch, and I went with her to eat lunch as well. I believe this was a mistake on my part because she didn’t really invite me to come, I just sort of came along. I kind of realized my mistake afterwards. During lunch, the conversation was casual: she was laughing a lot and I got to know more about her. After she ate lunch, she said she had to get going, so I just let her go and I went to do my own stuff.

The 3rd week: I was planning on asking for her phone number. She initiated conversation again, but as we were going to the usual place we sit, she said she had to go somewhere else. I just told her “Im going this way” and just waved her bye and left. I wasn’t planning on making the same stupid mistake and follow her again to where she was going.

My question is, what was her deal with her having interest in me twice and then just leaving me afterwards? And where did I screw up?