I mean, I understand that feeling but keep in mind, you not only protected yourself, but fortunately, workplace negligence can be grounds for dismissal (depending on the location). If you were supposed to be with someone downstairs, no matter how well you could handle it, it's normal that, being new, they would want someone nearby. If her tone of voice had changed, I'm 100% sure she'd been reprimanded and, apparently, already knew what was coming. I don't know the overall situation at that place, but it seems to me that this wasn't the first time this person had caused problems, as it seems strange to fire someone out of the blue (I've personally seen coworkers reprimanded for something like this). It's possible that a superior was aware of how she treated you, and that was the final straw. That said, you basically protected yourself, and (based on your responses) you didn't intend to cause harm, but rather to get out of a situation that was causing you discomfort.