Some people are just like this and there is little point cogitating on what you may have done to deserve it, it is her, and she is best left to get on with it.
Be cool, be polite, but stop giving it any more thought or attention. You can't change her and it is unlikely that you are the only one she treats that way, stop trying to work it all out. life is too short to take any responsibility for unreasonable people.
I wouldn't bother with any gestures like a Christmas card that cost money, I post my season's greetings on FB now and save a tree.
Don't put yourself in the position where she can snub you but just see who she is and let her deal with it.
‘Nudinits’ in a shop window - a charity fund-raiser, required to cover up!