How to self-advocate after rotation rejection?
If you are in good terms with your PI and you have weekly meetings with her, it is probably best if you bring up the topic in your next meeting. If "lab politics" are really involved (which need not be the case), it is unlikely that she will provide more information through email.

If she had told you that she would have funding and space for you all, it is perfectly reasonable for you to ask what changed, so she should not be offended by the question.

Regarding how to phrase the question, I would be moderately direct but, as most "conflict management" courses suggest, prefer the use of "I-messages" instead of "you-messages". Something along the lines of:


  I would have liked to be able to stay here, is it really not possible anymore? And if it is not, do you have any suggestions as to how I can do better elsewhere in the future?