Yeah I don't know either.
Anyway here's an interview about it. Don't read it if you don't like spoilers or shit like that, it doesn't seem to have any, but Ive not read it all. Its basically just talking to the actress who would've been playing that character
http://www.mtv.com/news/1856411/game-of-thrones-lady-stoneheart-brienne-pod-season-5/