It can become chronic. I had it for 18 months and was coming to the conclusion I would never compete in an Ironman distance race. None of the doctors advice or standard treatments were helping (rest, Ice, compression, anti-inflamitories, orthotics, shoes, etc). There's only so much rest you can afford before your fitness drops off! When surgery was threatened, I got serious about finding something else.
Julie Donnelly's ("The Pain-Free Triathlete") self-treatment techniques worked for me. You can read up on Achilles Tendonitis at her forum at
www.julstro.com/18.html The lady is THE expert on repetitive strain injuries. She also answers all the posts herself. Look into it and good luck. You don't want this to hang around and get worse. BTW.. thanks to her, I have managed to compete an complete two ironman distance races so far (OK, so IM Utah doesn't really count).