UPDATE: Somehow fixed itself. While the phone was trying to sync my photos I went to Settings -> General -> Usage, as gkpm recommended. At first it still erroneously showed only 3GB of space avail. with only Apps listed underneath, music and video "no data." (NB: There was nothing listed amounting to the mysterious extra 13GB of 'other' that could be deleted) Then it changed, on it's own, in the blink of an eye, to what it should be.
The sync that was going on was stuck, so I cancelled it, but became stuck canceling.
I then clicked the disconnect button on iTunes but phone still showed it was syncing, so restarted the phone.
All capacity figures are correct now and phone is back to normal AFAIK.
Hope that helps. Good luck!