Unfortunately, there is nothing you can do to protect yourself, unless you have a written employment contract. Without an employment contract, you are an employee at will; an employee at will may be terminated--or anything less than terminated (e.g. disciplined, written up, chastised, etc.) at any time, for any reason. More generally, the law does not protect employees from being bullied, or require employers to treat them fairly or professionally. Your best recourse may be to seek other employment.