Two possibilities I can think of:

The first is she thought you were great friends and was very excited to hang out outside class. When you were gone, she told some other friends about the plans. The other friends informed her that you probably consider it a date and just copped out by not calling it a date, hence tricking her into thinking you are hanging out as friends when really that wasn't the intent. She feels betrayed, has righted her wrong, and is not doing anything to lead you on now.

Second, she was in the limbo of 'is it a date? or not a date?' and getting flustered/excited that maybe it could be a date. When you came back with a hicky, she decided 100% it's not a date cause you have someone else. Her hopes got crushed, all excitement died, and now she doesn't want much to do with you.