Windows C: drive bug (?)

Hello

My problem:

My space (C drive) is sometimes full and sometimes half full! Why?
I didnt install anything...
When the workspace is half full, and I restart the PC, it looks like this:

And sometimes it looks like this:

Im sry for my bad English :)

That is not surprising at all.
If your Windows 10 has a version update then around 20-25 GB is used for maybe 2 weeks until the old Windows 10 version files are deleted.
You made the (C Partition) rather too small, I would have used at least 200 GB.