Why is she ignoring me all of a sudden and acting strange?

I would say try going for it again! She liked that you said hey first probably because you were giving her a sign that you want to talk to her(rather than her having to start the conversation) and guys should be the initiators anyways! :P And maybe it just feels like you're talking to someone else because she finally realized that she does like you and so talking to you might make her somewhat nervous(not in a bad way). And no about the period, if that were the case she would probably be acting moody and possibly short-tempered, although not all girls react the same to that time of the month. Good luck! :)