Mac Hard Drive Storage Decreases even without downloading anything

I have a 128 GB hard drive on my macbook pro retina 13". My mac storage hard drive keeps decreasing for no reason, even when I do not download anything. This still happens even when I don't use any other application and when I use Chrome in incognito (so nothing is being stored in cache).

Here is how the pattern looks like when I did not download anything at all, did not use any other application except for chrome and only used Chrome in incognito. first day:

More days later:

As you can see from the images, I did not download anything, even if I did it was probably a Word document or a PDF file. And it still increased 10 GB on the Apps. Just wondering why this happens and how can I prevent it from happening? If this is all a glitch, then how much GB do i actually have left for storage? Finally, why does the total number of GB used from apps, photos, movies, backups, and audio do not add up to the remaining GB left? Like in the last picture it says I have about 52 GB free and the total used up is around 110 GB. Since I have a 128 GB storage, shouldn't it be 10 GB left?

Also I have no anti virus running. I don't even have Dropbox and Google Drive. Also, I turned off photo sync from my iPhone. Once again, no apps are running except for Chrome, iMessage, and Skype.

Update: every time I restart my mac the disk space comes back and then it gradually increases again.

Could be anything. App updates, new apps, a virus that downloads stuff for you, automatically installed software, or a host of other things.

If you did have an application on your Mac like Dropbox or Google Drive, (either of which could automatically be downloading stuff onto your computer) it could be that, but you do not.

Here's what you should do: download an antivirus app (like BitDefender or Avast). Run a full system scan in whichever you download and make sure no viruses come up, as some malicious software could be doing this.

Then try downloading Grand Perspective, which shows you everything that's taking up space on your mac. Run it through your entire hard drive. See what comes up, and save your scan; then try scanning again when your available space decreases again.

Please report everything you find out in a comment below so I can further assist you!