It might be the cranial cruciate ligament that is torn, which would cause the leg to be held up a good percentage of the time.
Topics of Interest > Cranial Cruciate Ligament Rupture
Or it might be a patella luxation.
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Either way, surgery is the only option if she is holding the leg up. Find a good board certified orthopedic surgeon to perform the surgery. I would not let a regular vet do it in case something arises and they need to change the surgery or make quick decisions.
The luxating patella is quite common in small breeds. it can be genetic or caused by a trauma. The rupture of the cranial cruciate ligament is usually caused by trauma.