4s space keeps disappearing

Hi all,

Iphone 4s with about 9.6 GB free. Space randomly keeps disappearing. Discovered this trying to take some pictures of the kids. If I reboot the space reappears, yet eventually disappears again.

Did a restore from icloud and the phone showed the correct free space, but it eventually disappeared. If I look at the usage in settings I see 9.6 GB free and 3.6 in use. At some point it changes to 13.2 used space, 0 free.

Any ideas on what this could be?

Thanks!