Have you checked whether the swelling is just around the ankle or if it's more spread out across your whole foot? Because honestly, it's usually a combo of several things - a poorly fitted boot (too tight in the wrong spot or too loose), repeated minor sprains you're ignoring, and muscles/ligaments that aren't strong enough to hold up for a full day on the slopes. People usually fix this in three steps: first check your boot fit with a salesperson (not just the size, but how it actually feels when it's on), then do some strengthening and balance exercises before the season (super helpful), and if it keeps happening, see someone who can check whether you have a minor sprain or something more serious.