This touches upon several different areas. If you are a New York City employee, you may have certain protections based upon the nature of your "victim status" and your employer's perception that you may suffer from an addiction. Further information and details are needed to give advice.
What was the nature of the case you were testifying in?
Why weren't you honest with your employer about talking to the U.S. Attorney? Were you subpoenaed or testifying voluntarily?
This answer does not constitute legal advice and you should contact an attorney to confirm or research further any statements made in this answer. Any statements of fact or law I have made in this answer pertain solely to New York State and should not be relied upon in any way in any other jurisdiction.