Sometimes Windows 10 shows me the wrong disk space since upgrading to v 1803.
I have 18GB free on C:, but after some time it shows 5GB and the low disk space notification pops up. After I restart my PC it gets fixed.
Windows is doing this randomly.
Updates maybe, could be not enough ram too, the computer will cache the data in file but still, 13 GB is a lot.
Try this program,
When you install it be carefull not to install dash freelane program by unclicking the option.
full scan your pc, all the useless cached files should be found and give you a bit more space
Plus the program has a cpu/ram meter so you will see if you are needing ram.
After you are done with your problem, uninstall it. reinstall it when you feel the need. Rince, wash, repeat!
