This sounds like basic overuse inflamation of your achilles tendon. Ice it and take over-the-counter anti-inflamatory pills. Try to get any swelling down quickly.
As you recover, consider mixing in trail running (or any softer surface than roads or sidewalks). Your legs will thank you.
For detailed info, go to www.runnersworld.com. Then select 'Injury Prevention' and specify 'Achilles Tendonitis'.
