Welcome to YorkieTalk.
If her knees are popping, it could be luxating patella.
You can give her glucosamine (my first choice is Cosequin) to help prevent further damage but depending on how bad it is, some dogs need surgery.
Since this just started, I would ask your vet to take a look and talk to them about supplementing with glucosamine. Does she ever carry her back legs or does it ever look like she is skipping?