I have had this problem as well, but before the new update came. this happened a few months back.
try going to setting and looking at applications that seem unusually large or questionable.
setting > general > storage > manage storage
I noticed one app the was doing this. i was told to delete and reinstall, which actually helped. and it happened again a few weeks after, and i did the same thing to fix it... afterwards never happened again.
if it's not an app problem, it might be the photos or camera. the new setting of the photo is it captures before and after the photo itself, which I've noticed takes up a large amount of space, this can be disabled in photo settings.
try finding the cause of what is consuming the space, usually if you find out, there is something you'll be able to do. if not, try restoring to factory settings, after backing up of course, i know the feeling of losing all date, not pleasant.
then using first without your data, if nothing happens, then might not be the new update. if there is, better talk to apple support directly.
but if nothing happens, then you restore your data, and the problem persist, it might be an app that was updated to accommodate the new iOS. it might be eating date, with out reporting the use of this to setting. sometimes, its a third party problem. that was the case with me.