How much space is being used? If its not a lot it could be your log files.
For instance your /var/log/messages may be adding a MARK every 20 minutes, or as you reboot adding boot information like mapping the RAM and logging various drivers and services.
I'm not sure about Fedora, but most distros wipe the temporary directory on reboot, or keep it in tmpfs.
If it's a bunch of data, you can use the du command to see where the space is being used. Here is a quick command to list the top ten directories by size:
du -cks * | sort -rn | head -11
If you issue this command in the / directory it will let you work down the tree to find what is eating your free space.
(you probably want do to do this as root)
cwd # cd /
/ # du -cks * | sort -rn | head -11
du: cannot access `proc/8637/task/8637/fd/4': No such file or directory [don't worry bout these errors]