At the beginning of this lesson my ranking was 63%. I made no mistakes in the lesson and at the end my ranking was 62%. I'm annoyed. Why?
The fluency rating is very inaccurate (there are several threads about it; you can find them by entering "fluency duolingo" in a google search).
Just enjoy your lessons, be proud when you make no mistakes, pay attention to the mistakes that you do make -- this will ensure happy progress (in my experience) :-)