Incorrect Disc Space Shown

A couple days ago, I find out that I have at least a GB left on my computer. A couple minutes after I check, I check again and now it's at 400mb disc space left! And so I delete 3GB worth of files and check again and shows that there's 3GB free space. The next day, I check it again and find out that the free space went back down to 400mb. Not only does it show that it's at 400mb, but the colors for Free and Used have been switched around. I go onto my other computer to see how much space is left on the one that I'm currently using and it still says 400mb.

So all I'm asking is if any of you guys know what the problem is.