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It could be that her ears wear played with too roughly and her cartilage was broken in her ear. We have a tendency to play with our dogs ears and with such a tiny pooch her ear's cartilage could have easily been broken. You can have the vet check this out.
Also, this can sometimes be a symptom of losing their baby teeth and once they are done teething the ear may stand back up.
It could also be that there is something in the ear or somethin is bothering it, possibly and ear infection. You can have your vet check the ear out to determine this as well.
Otherwise, each dog's ears are unique and though it may be standard for their breed to have straight ears every once in a while one will stray from it's natural design and flop over.
Hope this helps and if I have answered your question please accept my answer!
It depends on what is the reason for the droop.
If it is broken cartilage then it is not likely repairable but a vet can determine this for you.
If it is because of an ear infection or irritation once it is treated it will stand up on it's own.
If it is teething it will stand back up again after she is done teething.
If it is genetics there is a way of taping the ear to "train" it to stand up again.
Having a vet verify the cause of the droop will help you to determine the permanancy of it and what steps to take to correct it if possible.