18 GB is the minimum you want at any time.
Aee where it is all being used:
OmniDiskSweeper (free) - http://www.omnigroup.com/applications/omnidisksweeper/download/
OS X Lion: About Time Machine's "local snapshots" on portable Macs - http://support.apple.com/kb/HT4878 - “You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed."
Hidden local backups with Mac OS X Lion filling up your MacBook hard disk! - http://blog.valerio.de/post/19238158977/hidden-local-backups-with-mac-os-x-lion- filling-up-your - Time Machine using local drive to store backups when external drive is not available.
Omnidisksweeper is not showing you where the disk space is going? Have you tried it?
Have you restarted the computer? In Safe Mode?
Mac OS X: Starting up in Safe Mode - http://support.apple.com/kb/HT1455
What is Safe Boot, Safe Mode? (Mac OS X) - http://support.apple.com/kb/HT1564
Safe Boot takes longer than normal startup - http://support.apple.com/kb/TS1884