A "trick knee" is a medical condition known as a luxating patella and your vet is right that these can come and go with time. One thing to mention is that your vet should be able to cause the patella (knee cap) to pop into and out of place if this is a luxating patella issue. There also tends to be a correlation with the 'ease' at which this happens and the grading of the patellar issue. In many cases, the dogs may require having surgery to normalize the structures and keep the patella in the right location.
There's a super comprehensive page here on this condition: https://www.acvs.org/small-animal/patellar-luxations
And video of a similarly sized dog with a luxating patella that's holding her limb up (your girl is apt to look like this if this is, indeed, the same condition): https://www.youtube.com/watch?v=DfPgEAzBhQY
The shaking, however, makes me worry that she's reached an age where arthritis is setting in and she may need to be placed on daily NSAIDs. This is actually quite common not only for dogs as a whole (especially as they get older) but dogs who battle issues like luxating patellas. It would not be a bad idea to have an x-ray performed on the knee, her hips and spine to see what conditions may underlie. Alternatively, a few days on NSAIDs (with or without higher grade pain medication if needed) may be, to quote a phrase, just what the doctor ordered. If she has no other medical conditions, you can also try a dose or two of aspirin but this is NOT for long term use as canine-safe NSAIDs are much safer and offer a lower risk for her long term. My favorites for tiny tots such as herself are Metacam and Rimadyl (one or the other will be selected). These are easily dosed, flavored and dogs do well on them overall. If she's a picky eater already, the metacam would be a better choice as it's liquid and can be placed directly into the mouth or mixed into a small bit of food. Aspirin dosage: http://www.veterinaryplace.com/dog-medicine/aspirin-for-dogs/
If/when she becomes more painful with time, pain medication can be added into the mix. For a little girl like her, tramadol is often used in conjunction with NSAIDs as needed.
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