Whats in your personnel file? Ask for a copy of it. If its in there, then yes you have a slander case and a good one.
Also, in addition to the slander case, you should contact the inspecting authority - you likely cannot be fired for talking with them. The inspecting authority may have the power to have you re-instated.
And you may have to go through the authority's administrative processes before you could file a retailiatory discharge civil suit. Depends on your state etc.
See an attorney ASAP; there may be as little as 30 days to file any type of complaint.

