log out of itunes store in settings, wait 10 minutes, then log back in. your storage should go back to what it should have been all along. there is an itunes media caching bug / storage leak in ios 8 (all versions so far), and apple for some reason has decided it isn't worth fixing.
you may have to repeat this again if the storage fills up again
i would urge all ios users to try this trick before spending a ton of time on a full backup/restore
if this trick doesnt work for you, though, a full backup/restore is probably your best bet - and be sure to select the option to encrypt the backup because it will include apps and passwords and make your restoring that much quicker and easier