Hurt ankle when running

Today I decided to run a different course and ended up getting lost and running twice my normal time.  I ended up running up and down some extreme hilly areas and, some gravel and grassy areas.  During the run the only thing that bothered me somewhat was my knees when going downwards.  When I was finished I felt great!  However, when I got home I sat down at my computer for a while and when I went to stand up I had extreame pain on the outer part of my left foot, below the ankle.  It has been about 12 hours since my run and the pain is still pretty intense while walking.  It doesn't bother me at all when I am sitting or when I touch the area. Any thoughts as to what I may have done to it?