Could be a number of skiing and non skiing reasons. Maybe your boot is too slack or too big and not providing ankle support. Backseat skiing can do this, but its usually around the mid shin. Sounds more like boot missize leading to ankle pain. I did some damage while big mountain skiing in park full tilts; my ankles blew up at the end of the season. Pressure points can do that too, if you wear your boot long enough.
Off skis reasons can be lack of warming up before a running sport, bad walking shoes, walking in pavement for a long time and the list goes on.
Try an ankle brace on the off time to let the inflamation go down a bit before going back to skiing.