If she wasn't so shy and introverted, or arrogant and off the charts, she never took the initiative to find you, which means that she had more interesting (meaningful) things to do than chatting with you at that time, and she certainly didn't take the initiative to like you. At most, this kid is quite good (drop ~ good person card).
Warming up at school may be because some things can't be done, which means what you did in the dormitory mentioned above. Since I can't do what I want to do most, why not settle for the second best, chat with you and walk back with her, just to relieve boredom on the way. She may also take it for granted.
Then, you packed up in advance today, and she has been with you. Seeing you packing, I think I should catch up quickly. What happened? You left without saying anything ... How embarrassed was she? Maybe angry? Some girls will feel that they have done something wrong. I don't know about her, but it depends on her usual personality. ) Are you angry? (I am very wronged. Then she must let you know that she is angry, it's no use following you, and you won't look back at her or anything. Of course, she must walk in front of you (mind you, she is angry that you forgot her). Then walk away and dump you (demonstration or something), indicating that she doesn't need to be with you and ignore you (gambling).
But be sure to apologize and coax. You can't condescend, you can't act like she is unreasonable, you can explain the reason clearly, you can make it up. What is wrong with me? I was very unhappy, and I fell into a panic. I left with my brain, Barabara. Sorry, it won't happen next time. Speak more, speak softly and use more modal particles.