Thank you both for your replies!
Interestingly, 1 month ago (before I reinstalled a fresh copy of windows 10), when this issue happened, I did notice a couple file folders directly inside the C: folder which I attributed to ccleaner after it not finishing. I deleted them and all was well.
Since reinstalling windows, now when I look in C: I don't see those folders.
I figured out the command to search in file explorer to find folders larger than 4GB (size:gigantic kind:=folder) but surprisingly, nothing came up!
<See screenshot>
Any idea what I can do next to solve this one?