"I had no idea you were good!"

I started taking singing lessons senior year of high school, and I loved it. At my very last lesson, before leaving for college, I sang my heart out on a song we'd been working on for a while. My teacher was really impressed, which made me so happy, but then she said, "Whoa, why didn't you sing like that the whole time? I had no idea you were good!" 

This made me so sad, and her comment has stuck with me. If she thought I was bad this whole time, why did she just passively keep it to herself? Why didn't she try to actually help me improve? 

And I hated that she made her comment in a tone that implied that we were both surprised that I was actually good, and that the general understanding was that my default was "bad."