When a dog holds their leg up, it is called a root signature sign. This can mean there is a problem with the vertebrae in the neck. She could also have an injury to nerves in the area. As you mentioned, this could be a shoulder injury. You did well to check for any foreign objects in her foot. There is not much you can do for her at home. She will need to be seen by the vet since it has been going on for 2 days with no improvement. They will likely x-ray the limb to check for obvious injuries. If it is an injury to her neck, they will do a myelogram to check for compression of the spinal cord. Until you can get her to the vet, keep her quiet and strictly rested. If you notice progression of the signs, she needs to go to an emergency center. Hope this helps.