Hi All!
Wanted to share my experience! Yesterday I updated to 8.1, everything was perfect. I went to bed, and disconnected wifi and data. Today, when I woke up, my 5S had a message on the lockscreen telling me that I had run out of space 😱.
Then I went to prefs panel, usage, and there you go, 0kb free space, wow. I deleted a game to free up space, and the space miraculously was indeed filling up the free space I just got. I have no strange space info on the prefs pane, so I just left it that way, waiting to restore my 8.0.2 backup. iTunes told me that I had 15 gb of documents space in my iphone, which was not true.
So two hours later, I checked again my 5S, and again, miraculously, I had 15 gb of free space.
I have no clue guys.
EDIT: Hi there, my free spaces is being cut down again. Now I only have 9gb free, apparently whithout any valid reason, sigh.