Go see a vet or a orthopedic vet by: Stefanie - JRT mommy
My JRT Penny does the same thing. She was recently diagnosed with Patella Luxation. It's a genetic defect. Unfortunately it is not uncommon in JRTs and other small dogs. You can have a surgery done to correct it. One surgery requires shaving some of the bone. The other option is a tightening of the ligament that is loose. The surgeon will decide which will help the dog more. It is better to have the surgery done earlier than later because it prevents arthritis from happening earlier.
I'm sorry if this is bad news. I cried the whole day when I found out. We're still trying to figure out what to do. We have been quoted $1200 by our regular vet and the orthopedic surgeon would probably cost more than $4000. We also have the option of going to Texas A&M which is the largest vet school in the US (or so I was told).