What your girlfriend is exhibiting is withdrawal symptoms. This is commonly displayed by dumpers.
She dumped. You acted cool. You were calm. You told as she wishes you'll respect her decision. You moved on with your life. She expected you would beg. You would plead. You would ask her for reconciliation, that she would come back to your life.
Your lack of response and that you seemed to deal with the breakup in a positive way got into her. While you moved on, she didn't. While you healed and recovered, she hasn't yet healed. Thus, the emotions she's displaying is, as a result, you're well off while she's struggling with her emotions. The 'I miss you,' the insulting and the blaming are a result of the fact she hasn't gotten over the breakup despite the fact she's the one who ended the relationship. As of her telling you she has a date the next day might be true or not. In most cases it's not. She wants you to know she's doing fine, she's over the relationship and you, she has a man in her life thus everything is going well in her life which isn't true.
I call it withdrawal effect because she's begun exhibiting signs related to a breakup or end of a relationship or the fact she's no longer in relationship with you.
I hope this lengthy answer provides you with an idea what goes on on a dumper's mind though it's not exhibited by all dumpers.