There is usually a few reasons for pain in the ankle when riding. Some of them are bracing the ankle meaning putting too much weight into the stirrups. While you do want your heel down you do not want your ankle or leg braced. Another cause is rolling the ankle under . . . putting more weight on the outside of the stirrup than the inside. Try angling the stirrup so the outside of the stirrup iron is closer to the toe and the inside of the stirrup iron is closer to the ball of your foot. The double jointed stirrups also tend to be helpful for people with back knee and ankle problems, because they offer some give and shock absorption.