I am a first year PhD student. I just finished my first rotation lab and absolutely loved it. I met with my PI every week, and I never felt like she had a problem with me. Though I never had a frank discussion with her regarding the fact that I wanted to commit to her lab (big mistake, I know), I heard from other students who spoke with her that she wanted to make sure she had enough space in the lab for me and the other rotation students had we chosen to join. She also promised us that she had enough funding so we wouldn't have to compete with one another. I really thought I had found my lab home until....
I received an email from her yesterday telling me that she couldn't keep me. The reasons she gave me seemed flimsy (i.e. I wasn't enthusiastic enough....when literally anyone else in the group can vouch for my enthusiasm!) and contradicted what she told me at the start of the rotation (ie. she said she wished I had gotten further in the project, when she explicitly said multiple times that she didn't care how far we got through our rotation projects).
I hate confrontation, but I want to be a self-advocate. My biggest regret in undergrad was not self-advocating enough. I feel like I lost a lot of wonderful opportunities because of my fear to approach professors. But this time I want to stand up for myself, try to get a chance to defend myself and ask for legitimate reasons why she suddenly rejected me. And of course, try everything I can to try to change her mind. This is my dream lab, and I want to be brave enough to fight. I know I'm stepping on some annoying lab politics by doing this, but how should I go about addressing my concerns with my PI while not offending her?