I was terminated for negativity without any disciplinary action or ability to defend myself.

I had made some comments during a training(which I apologized for) in a joking manner that were taken the wrong way and have voiced my concern re: the continuously increasing workload and the following day I was terminated and was not given any warning nor was I allowed to state my side and/or concerns, I was told I was too negative and although I an a greta nurse and in no way does this have anything to do with my job performance the district will not tolerate my agressive attitude. I have never before this been disciplined or in any way written up for this "attitude problem" but things were mentioned that I was inappropriate on other occasions, but htis was never addressed prior to this in any way, I was given a sealed letter that I could not open and walked out of my office. I feel that I should have been talked to and at the very least put on some sort of improvement plan, but not terminated with no disciplinary action in process. Please advise.