Sarah, you could also have a very high tolerance of pain (like my wife) or be extremely flexible too. I think it's a good bet that the lack of uric acid in your joints has a lot to do with it, could be a lot of different factors working together.
Kev, as far as the uric acid idea goes, I only just read it in a yoga book, so I am pretty much in the dark. Maybe find a competent yoga instructor or a nutritionist and find out what they suggest. Glad to be of some help.