Employment Law Questions? Ask an Employment Lawyer.
you will need to check your employment handbook on policy and see what it says about an "at will" employee - this is generally the case. this means that an employer can fire you at anytime for cause or no cause.
the exception would be if you have signed a contract for a specified period of time
or if there is nothing in the handbook or it was never discussed that you can be fire at will - in otherwords, if your employer has something or there is something in the handbook and policies that infers as long as you do a good job and produce you will not be fired.
if that is the case, and you can prove that, then you should consult with a local attorney about a possible wrongful termination lawsuit.