Perhaps it's you -- then again, maybe it isn't.  This series of articles from Fedsmith.com gives you some idea of what a good supervisor will do to communicate his/her expectations to the employee. Is your supervisor doing this?

When Bad Supervisors Happen to Good People: The Price of Poor Supervision (August 9, 2007)

 The Best Performance Discussion Guidance I Ever Borrowed  (January 18, 2008)

The Best Performance Appraisal Advice I Ever Borrowed (Part II) (January 27, 2008)