Help with teenage girl?

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I went to get coffee with a girl I used to date back in middle school, a few days ago. She said that she wanted to do something again. Yesterday I texted her seeing if she wanted to go to a play at my school(we go to different schools) and she didn't reply for 6 hours. I went on Facebook twice during this time and she was on Facebook on her phone both times. Then she texted me saying she was sorry for not replying, and that this weekend wouldn't work, but that she wished she could . Why did she do this? What should I do?