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Unfortunately, employment in the state of Indiana is "at will" absent an express agreement to the contrary. At will employment can be terminated for any reason not amounting to discrimination on the basis of a legally protected trait (race, religion, gender, etc.) or retaliation for engaging in certain forms of legally protected conduct (filing a wage claim, taking FMLA leave, etc.). It doesn't matter whether the basis for termination is fair, reasonable or even TRUE.
Since employees can be terminated "at will," the law does not regulate evaluations. Indeed, it would make little sense to say that employers have to follow specific protocol when evaluating employees when they can simply fire the employee without offering a formal evaluation at all.
So, this is really not a circumstance in which you have any legal recourse or protection. That is the hard truth, but you deserve a straightforward response. All you can do is attempt to address the concerns about your performance that your employer has raised, however unjustified they may be. If you are terminated, you will be entitled to unemployment benefits unless you do anything to intentionally provoke termination, which would disqualify you on grounds of misconduct.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.