You lost 125gb overnight?
Have you tried Disk Cleanup? -- under Accessories - System Tools.
If all fails, I would first manually search large files with explorer but add terms "When was it modified" -- 2 days ago?-- and specify size at least 10MB. The missing gb might be due to countless small files generated by premiere (if that's the case).
Another option is you can check each folder "properties" (right click on the folder) and see which folders have large size, then go to each subfolders to check.
Edit: Try also chkdsk under command prompt (Start - Run - cmd) and see if there is lost clusters etc. The command is "chkdsk x:" where x is the drive you want to check.
Let us know how it goes.