Employment Law Questions? Ask an Employment Lawyer.
In employment law, if you have no employment contract specifically stating that you can only be terminated for cause, your employment is 'at will' and can legally be terminated at any time, with or without cause. The employer does not have to have had prior complaints or a history of documented issues to terminate someone.
You don't have to be called into the office or written up either. The employer can just make a decision without going through any sort of formal process.
Unless you can allege that the only reason you are now being released is discrimination against your race, religion, gender, age, disability or recent FMLA use, there isn't anything illegal in the facts you've mentioned.