If you normally use Time Machine to backup to an external drive or NAS, and you leave TM on while you are mobile and not connected to your backup drive, you may have some local TM backups taking up disk space. There is another thread on this forum discussing this issue.
The recommended procedure on that thread was:
You are not freeing up disk space when deleting files because of local (as in "on your hard drive", so not on the Time Machine drive!) Time Machine backups, a new feature introduced in Lion. This feature is per default activated and can, unfortunately, not be disabled in the Time Machine menu. But: you can safely disable the local backups using the following terminal command:
sudo tmutil disablelocal
This may be the problem with your disk space usage.