Its very hard to say without seeing you what the problem may be.

My first thought from your post was that when training for the marine, what was your training schedule and what sort of shoes were you wearing.

When does it actually hurt you, during what activities? And your certain you didn't twist it? Also no previouse ankle injuries on that side?

Sorry for all the questions, just trying to get a better idea of the problem. If you could post a full history that would be great.