Could be a high ankle sprain. Unless you stay off it, it'll take forever to heal. You can play through it, but I wouldn't recommend it unless you have shoes on. I got mine on Drop Out 6-Panel.
I've heard that this type of injury can require surgery if you keep playing on it (like NBA players).
BUT let's hope you're just pounding the pad too hard. In which case...put on some shoes, or put a cushion under (don't know if your pad is hard or soft)...or just play less?
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Pain is temporary...DDR is FOREVER!
In the Mother Land! Nonstop Bemani and games and...teaching English.