Why Won't My Old Friend Answer Me on Facebook?

I had a best friends for like 12 years, we got in a stupid fight almost a year ago. She was talking to one of my other friends and told her that she would talk to me and be friends again but she doesn’t have any classes with me. I decided to message her on facebook and I wrote that I wanted to be friends with her again, and basically asked her to message me back if she wanted to hang out/talk again. Its been like 3 days and I know shes been on facebook since and she hasn’t answered me. And I know shes not mad at all because we did talk a while ago and she said she had NO hard feelings at all. Do you guys have any idea as to maybe why she hasn’t messaged me back, and what I should do without being pushy. I want to give her space and let her think/decide.