Do I have a case against my employer?

On tuesday, I took a five minute break in the morning. Our company allows fifteen minutes in the morning and fifteen after lunch. The office was staffed when I walked out to take an important phone call. That same day I sent an anonymous email to the company owner regarding some illegal practices my boss was doing behind the owners back. After he was confronted with the information, he wrote me up for taking my break, stating the office was unmanned. There were two other people in the office the entire time I was gone. I refused to sign the write up because I am entitled to a fifteen minute break. I did this calmly, but then was fired for insubordination for not signing the paper. I talked to another manager, but he said that because there was a rumor about me talking badly about my boss, I would not be allowed to have my job back.