Welcome to Justanswer! I am Dr. Altman and happy to assist you both today!
I am so sorry to hear about Pixie. The primary concerns are a patellar luxation or a cruciate tear with her breed and symptoms. I would advise a veterinary exam because a home evaluation will not give this diagnosis, a veterinarian needs to check the stifle for drawer or movement which cannot be done without an experienced hand versus the kneecap floating which palpates very differently. Often with palpation and manipulation at home you will not see any indications of this condition
Here is more information on cruciate tears in dogs:
And for patellar luxations:
Until you are able to have an evaluation, please limit activity as much as you are able. Rotate warm and cool compresses on the leg in ten minute intervals, and massage from toes to hip and back down again frequently to stimulate blood flow therefore healing
Until examined please lift to go potty and leash only to minimize running and jumping which can cause further injury.
I hope this information makes sense. It is possible that this is a simple muscle injury but I suspect you would have seen some improvement if that were the case in this length of time. Please let me know how else I can assist you both today
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