I do this, too, squiggle, I do it all too often....interpreting a simple comment as having something to do with something bad in me. And often, I do the same sort of shut down routine.....but actually, your T didn't let you completely shut down or shut her out, did she? So the good thing here is she saw your reaction and helped you see it what happened....it's good too that she was able to give you a reasonable explanation of why she said what she said, too, so you didn't just leave with that burning in your mind and not know why she said it.
I think perhaps your statement at the end about her having the power has something to do with your reaction to this statement? Maybe it seemed like a sudden exercise of power that caught you by surprise, or just a reminder of your vulnerability, a feeling of not being in control.....I don't know. I can't tell you why you do it because I haven't figured out completely why I do it.....but I know it has something to do with my feelings of inferiority(lack of self-worth, feeling like I must be unimportant...) or of not being in control or just simply misreading the other person's emotion or motive.
I wish you much insight with your assignment!