Use a disk space management tool to find out which folders occupy most of your disk space. Make sure to run them "As Administrator" so that these tools are able to see the whole content of your disk.
If you are not sure what recently ate up your disk space, use the print feature to get a printed report, you can use this later to compare it with your current results.
Even after emptying the recycle bin, the folder Windows System Restore may still eat up some of the disk space as it uses NTFS shadow copies to be able to restore previous states of the hard disk and the system configuration. Windows will free this space automatically after some time as it deletes old system restore point from time to time. You can manually delete system restore points in the “System” applet of the Windows Control Panel.