Your report against your co-worker was protected activity only under certain specific circumstances. If those specific circumstances are not the case in your matter, then your employer is not prohibited by law from terminating you for making the complaint. Complaints to HR are protected activity and cause a retaliatory termination to be unlawful if the complaint is about unlawful discrimination based on a protected characteristic such as race, gender, age, religion, etc. Other complaints about a colleague, such as about the co-worker's manners or personality or work habits or unpleasant and contentious conduct -- these complaints are not protected activity and a termination for having complained about these issues will ordinarily be lawful.
It is important to understand that unlawful is not the same as unfair. It is unfair that you are at risk of termination, but it may not be unlawful. You may want to take a long look at the Performance Improvement warning and make a strong effort to remediate your relationship with your manager. Asking for more details about the allegations may be a poor strategy for holding onto your job if your original complaint was not protected activity.
Good luck to you.
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If you have a discrimination theory about retaliatory discharge or hostile workplace conditions you'll need a lawyer. Religious belief, age, gender, race etc. all are discriminatory reasons for which you cannot be legally fired and upon which you might base a wrongful termination suit.
If you are an at-will employee, you can be fired for any reason or no reason, except unlawful discrimination. If you do not have an employment contract or union to represent you, your recourse is limited. Discrimination based on age, gender, race, religious beliefs etc.... may give rise to a different answer.
As Ms. McCall correctly notes, not all discharges after a complaint are wrongful. They are retaliatory and unlawful only if the nature of your underlying complaint was protected by law--such as a complaint about discrimination or a whistle blower complaint. I do advise that you speak to an experienced employment lawyer as soon as possible as you may have claims against your employer. Most of us, myself included, will provide a free consultation.
This answer is provided for informational purposes only and should not be relied on as legal advice. You should be aware that no attorney-client relationship is established through this answer and none will be established without a personal consultation and the signing of an engagement agreement.