Im sorry you are going through this. The girl you are/were seeing sounds a lot like me, sadly. I go from being excited to seeing someone I am dating, to not wanting to see them at all, just wanting to be alone. I don't know what to say other than give her some time. Don't call, don't contact her at all for awhile. I know when I am in a mood where I want to be left alone, I get very annoyed with someone calling/texting me constantly.
Anyways, good luck.