If you are a union member, your pay is controlled by your union contract. If you are not working under a written contract you are considered an "at will" employee. Employers of at will employees can make any rule and can terminate the at will employee for any or no reason except discrimination. Discrimination is legally by age, sex, race, ethnic origin, nationality, religion, disability, color, marital or military status. You should look at www.eeoc.gov for information on discrimination.
If you fee you have been discriminated against you can file a complaint with the EEOC and with your state's human rights office/dept. This is considered a lawsuit by the employer, and may make your situation worse at work before the EEOC can investigate it. There is no such thing as a complaint department in employment law.
You probably should contact a local employment attorney who can assist you and look into other options.