iOS 8.2 swallowing all available storage space
I've got an iPhone 5S with 16GB storage, and updated to iOS 8.2 the other day. Since yesterday, I've been noticing that the available storage on my phone is persistently at 0 bytes, and 'used'' is at 12GB. I know full well that the whole 16GB isn't available for the user, and 12GB is about right, but what I can't understand is that when I managed to upload a load of photos to Dropbox, then delete them from my photos gallery, it shows an amount of free space for a short while, then that disappears. It's the same with Apps, I've deleted a couple of large apps of 100MB+, but the available space is 0 bytes.
I've even uploaded a huge batch of photos to Dropbox, deleted them from 'photos', and from deleted items, and confirmed that the usage showed 2.9GB available space. An hour later, I try to upload another batch to dropbox, and it crashes repeatedly because something has swallowed that 2.9GB already.
When I go into Settings > General > Usage > Manage Storage, it shows I have about 1.6GB of photos and the sum total of every other app listed is somewhere in the region of 1.5GB. This includes a total of 153MB of music. I don't sync any videos.
So what on earth can fill up nearly 3GB storage in an hour when there are no apps open?