Reprimanded for discussing pay with fellow employees.

A few months ago I was reprimanded(written up) at my job for discussing pay with fellow employees. I had recently discovered that a new hires starting pay was double the hourly rate of many employees(myself included) that had been with the company 3 plus years. I indeed shared this information with those employees. My manager was notified by another employee about myself discussing wages with the employees and I was reprimanded, as she said what I was doing was against the employee code of conduct. I was also docked on my yearly review for this, influencing a lower pay increase due to the mark against me. Although this occurrence happened a few months ago, I would like to know if I would have a case against this. This manager has done many things to many employees, whom have reached out to management over her and nothing has been done about it, so I feel obligated to stand up and speak out if this was an illegal action taken against me.