Did the vet take any x-rays? 4 1/2 seems awfully young for arthritis to set in...I assume she is a healthy weight and normally active until now? You said it seems to wear off after a few minutes when she first gets up? It could just be a bruise but I would imagine that for it to bother her for so long, you would remember her initial injury. Could she have sprained it, gotten it caught in something? No lumps, bumps or anything unusual visually? What about her pads or between her toes? My dog Duncan cuts himself there occassionally and it's very difficult to locate.... What does she do when you touch it? I really don't know what it could be but if it was me, I would wait a day or two more and take her back to the vet and push him/her for a more thorough exam. If you're worried about it, it's something to worry about...Good luck and welcome to Pet Talk. I am sure there are many many others here who can be much more helpful than I...Good luck. I hope everything is OK!! : )