APFS decreases space randomly
In my experience, there are two major culprits for this unexplained disk usage:


Time machine
Swap


One possibility is time machine. If you open Disk Utility, you may see a section labelled "Other Volumes", these are likely to be time machine backups. If you do see this, then you can open a command shell and enter:

df -h


On my machine, I get output similar to:

Filesystem                                                             Size   Used  Avail Capacity   iused               ifree %iused  Mounted on
/dev/disk1s1                                                          465Gi  442Gi   18Gi    97%   5512606 9223372036849263201    0%   /
devfs                                                                 196Ki  196Ki    0Bi   100%       678                   0  100%   /dev
/dev/disk1s4                                                          465Gi  5.0Gi   18Gi    23%         5 9223372036854775802    0%   /private/var/vm
map -hosts                                                              0Bi    0Bi    0Bi   100%         0                   0  100%   /net
map auto_home                                                           0Bi    0Bi    0Bi   100%         0                   0  100%   /home
map -fstab                                                              0Bi    0Bi    0Bi   100%         0                   0  100%   /Network/Servers
com.apple.TimeMachine.2017-10-19-174031@/dev/disk1s1                  465Gi  428Gi   18Gi    97%   5078237 9223372036849697570    0%   /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Smakolyk/2017-10-19-174031/Smakolyk
com.apple.TimeMachine.2017-10-19-072101@/dev/disk1s1                  465Gi  423Gi   18Gi    97%   3858374 9223372036850917433    0%   /Volumes/com.apple.TimeMachine.localsnapshots/Backups.backupdb/Smakolyk/2017-10-19-072101/Smakolyk


Here you can see that I have local two time machine backups, and these will be using space. Apple have more information about local time machine backups.

You can disable local snapshots with:

sudo tmutil disablelocal


This should turn off the feature and free up the space. You can turn it back on with

sudo tmutil enablelocal


The second option is swap, this you can most easily see in /var/vm:

$ ls -lh /var/vm
total 10485760
-rw------T  1 root  wheel   1.0G  9 Oct 08:30 sleepimage
-rw-------  1 root  wheel   1.0G 18 Oct 14:00 swapfile0
-rw-------  1 root  wheel   1.0G 18 Oct 15:09 swapfile1
-rw-------  1 root  wheel   1.0G 19 Oct 21:25 swapfile2
-rw-------  1 root  wheel   1.0G 19 Oct 09:11 swapfile4


As you can see, I have 4GB in swap on my machine, and this is also taking up space on the disk.