That is pretty common. In fact, that's happened a lot with me as a high schooler, where I would know a few friendly people in a class, then when the semester is over, they become irrelevant in my world again.
If you haven't already, go look her up on Facebook, Twitter, Instagram, etc. You two know each other, so don't worry about coming off as creepy. Or if you see someone you know knows her, ask them for her phone number. And of course, there's always the chance you will run into her again.
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Early 20s male with Asperger’s and what feels like a mood disorder