Could be a number of things. Might just be that you are running more miles than previously and your body isn't used to it, have you upped the mileage in the last few weeks?
Are you stretching properly before and after your runs, its amazing how much this can help.
I used to get a similar pain in both my achillies to get rid of it when I was running I would try and kick toes of my feet up once I had pushed off to try and straighten my leg. Quite difficult to explain with words but I found it relaxed the tendon and helped.
Good luck, thou hopefully someone else with a bit more experience will post soon!