Why does caffeine dehydrate you?

This is purely anecdotal, of course. I had my teeth bleached by the dentist last Thursday evening and was told to avoid coffee & tea for several days to avoid staining. One day later I had terrible back and hip pain (I was expecting to get headaches!) that prevented me from sleeping, even after taking ibuprofin. On Monday morning, I still had the back & hip pain. After my first (and only) 20oz cup of coffee, the back and hip pain both disappeared.