I do not know the facts so I cannot advise you. You should consult your union or a lawyer immediately. In general, you should never sign anything that is not substantially true. Sometimes an adverse action document only requires a signature as to the fact that you were made aware of the contents, not that the statement is true. Without a copy of the document to read, no one could tell you what the effect of your signature would be. If you refused to sign, a supervisor may have noted that and signed the document. It will probably be placed in your personnel file. You are entitled to have a copy and should have been given one.
If you were taken to a place where you felt intimidated and interrogated at some length, the employer's action may have stepped over some line, but again the context is important. You should visit a lawyer to determine what your rights are.
Your organization may have a grievance process. It could be used to complain to higher ups about your treatment. Consult your employers policy manual or your employee handbook if there is one. If there is no such process, nothing prevents you from taking it upstairs if you feel it would do any good.
Any complaints to management by employees would probably be kept confidential unless they were needed to validate some adverse action against you. They could be anonymous.
In Montana, termination is allowed only for bad performance or a legitimate business reason. You are past the time when you could be terminated for any reason or no reason. Just cause would have to be established if you brought a wrongful discharge claim.
DISCLAIMER: The forgoing comment is for general educational purposes only, and is not legal advice upon which the reader may rely as the commenter has no actual knowledge of the facts of the case, has not interviewed persons or examined evidence, and has not researched the applicable law. The comment is based only on the facts provided, which are extremely limited, and may or may not be true. Complete defenses may prevent the success of any claim. Competent legal advice should always be obtained before taking any legal action or filing suit. Readers employ any information provided herein at their own risk.