Will an advisor schedule a PhD defense just to get rid of an underperformed student?
Based on what you say, I doubt that there are any issues other than getting yourself prepared. The comments here at this writing are all valid. It seems like she just deferred to your own judgement about scheduling but was giving you a way to extend prep time if you thought you needed it. I don't read anything negative in to that. 

Likewise, I can't see any professor scheduling a defense just to "get rid" of a student, whether a superstar, a typical student, or an underperformer. In fact, in the latter case, most would rather give the student time to come up to whatever standard is expected. 

Some professors, of course, when faced with a seriously underperforming student would be more likely to counsel them to leave academia and find a more suitable career before possibly facing a failed defense and then carrying that record for the rest of their lives. But there is no hint of any of that here. 

Some professors press their students pretty hard to finish, but because they judge the student ready to move on. 

But, you did ask, and you were adamant. You can live with that and get ready, or you can go back to her and say you misjudged and would, indeed, like a bit of extra time, if that is what you really want.