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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
The most common cause of this intermittent holding up of the rear limb in a small breed dog is a luxating patella. This is where the kneecap (patella) pops in and out of the groove that it normally sits in. It's very common in small and toy breed dogs to have this condition. Most dogs don't have any issues with it. When it occurs too frequently or it's painful, that's when it's a problem.
Here is a nice write up on the condition:
I would get her started on some pain meds to see if that helps resolve the issue. If not, then I would get a consult with an orthopedic surgeon.
I hope this helps, let me know if you have any other questions.