legalland, martha and scooter libby

I was a lawyer but obviously a lousy one. Because I still can't figure out why Martha Stewart was convicted.The story is she lied to an investigator about wether or not she had committed a crime. She denied that she had so she was charged with obstruction of justice. Was she supposed to confess to a crime when questioned ? If a suspect charged with murder denied that he committed the crime could he later be charged with lying? Isn't there a rule prohibiting forced self incrimination? This brings us to the Libby trial .He is charged with obstruction of justice in a situation where no crime was committed or charged to anyone . How can you obstruct justice in a case where there is no crime. Legal fictions are fun but only when Grisham or Gardener write them, and you don't get stuck with attorney fees, or sent to prison.