Hi, My name is XXXXX XXXXX I’m happy to assist you with your question today. I’m sorry to hear about your employment matter.
Generally, you have not experienced, as you have explained it, anything that is unlawful. Specifically, you have not mentioned anything that would trigger any one of the many employment laws to provide you some recourse against this treatment.
Minnesota like most states is an employment at will state - meaning you can be disciplined or terminated for any reason that is not otherwise a violation of law. These exceptions are the civil rights protections (e.g., age, race, sex, religion), violations of public policy (e.g., fired for attending jury duty, for refusing to break the law, for reporting illegal activity by the employer (aka a "whistleblower violation"), or having some contractual right to a just cause employment (meaning the employer cannot terminate you without industrial due process - which basically insures a fair and accurate investigation and decision). Without any of these exceptions, courts find that the employer has a legitimate business right to operate its business however it sees fit and the court will not second-guess the employer. These policies and decisions may in fact be awful and clearly not fair or even make good business sense. However, they are not ultimately unlawful.
In your scenario, it sounds like the boss's girlfriend simply doesn't like you or doesn't believe you are running the store properly. You haven't explained that the action is based on your civil rights, any law that you've attempted to enact or is a breach of any contractual agreement you have with the employer. If the employer simply desires to move his girlfriend into the position he can do that, and does not have to have a legal justification for the same.
If you believe you have any of the above-described possible exceptions to employment at will, let me know and we discuss the matter further. However, as you wrote your question, I do not believe there is legal recourse for you in this matter as it stands.
I believe this answers your question. However, if you need clarification or have follow-up questions regarding this matter, I will be happy to continue our conversation – simply reply to this answer. If you are otherwise satisfied with my response, please leave a positive rating as it is the only way I am able to get credit for my answers. Thank you, XXXXX XXXXX wish you all the best with this matter.