Hi and welcome to JA. Ray here to help you today.
You can argue the facts that you never agreed to cover those shifts.I also wonder if this could also be a smoke screen for discrimination.Often when they want to get rid of an older worker, or based on your sex or age they using this as a pretext.Filing a complaint with EEOC here is a means to protect you from wrongful termination and other adverse action.
You may also get a right to sue letter from EEOC allowing you to file a civil suit for damages from all of this as well including damage to your reputation.
If you believe that you have been discriminated against at work because of your race, color, religion, sex (including pregnancy, gender identity, and sexual orientation), national origin, age (40 or older), disability or genetic information, you can file a Charge of Discrimination. All of the laws enforced by EEOC, except for the Equal Pay Act, require you to file a Charge of Discrimination with us before you can file a job discrimination lawsuit against your employer. In addition, an individual, organization, or agency may file a charge on behalf of another person in order to protect the aggrieved person's identity. There are time limits for filing a charge.
Also if they terminate you afterward file another one claiming retaliation here as well.