Perhaps you had A LOT of data on temp files? You know, temporary downloads and cache and stuff...
Or, it might be that the system is not calculating things right. My GS4 sometimes has the tendency of being overly dramatic and says that I've installed 97GB worth of apps and media files... yup, the unit is only 16GB and the SD Card is 32GB. You do the math ('cause apparently the OS doesn't).
Did the "Space Available" section change, too?