My boss keeps threatening to fire me.?

I have been working with this company for about 4 years now. I was a sales rep then I got promoted to manager in Oct of last year. At the time of my promotion I inform my boss that I have legal matters that I take care off.on my days of. I passed my probation period of three months but on the 4th.month I got demoted back to sales rep. My boss stated that the reason behind this demotion was because of my legal matters that I take care of of on my days of. He made me sign a demotion paper but left the reasoning blank. I did not receive any warnings nor write ups before this taking place.I was a rep for a month and a half then got promoted back to management. Now him and his superiors keep threatining me to fire 2 months into my promotion again. I have documentation of everything. Should I sue can I sue?