Where I live, there's usually a time frame of a few weeks or months up to 6 months where you can be fired without any reason. As it was OP's first week, I'd assume it was something like that for her as well.

In many states in America, anybody can be fired for any reason or no reason. It's actually easier to get rid of people without cause because you can challenge an unjust cause in court. Working people here are screwed.

It is a rather medieval and backward system in many areas of the U.S.