"Other Storage"

Options
Hi everyone,
today i tried to backup my iphone apps from iTunes ( my iphone is running jailbroken IOS 4.2.1 ) and before backup i had ~ 3.3 GB of free space ( apps size was ~5,5 GB, pictures ~3 GB, and other stuff ~4GB ). So i started backup and left for few min, and after i came back my free space was 0 mb, and my iphone apps weren't sucesfully transfered.
So the question is: what the f*uck ? How did my free space got used by other stuff ?

reason for backup: im moving to ios 5 so i wanted to make a backup of apps