I figured out what I was doing wrong, I was downloading stuff to my firefox downloads and then moving them from there to my external HD, so now I'm just downloading directly to my external HD and my main drive is holding steady. I ran disk cleanup, cleared out my cookies and temporary internet...
I have an external hard drive that I'm downloading a bunch of media onto, and my main PC hard drive is supposed to have 321 GB of free space, but ever since I've been downloading all kinds of stuff onto my external HD my main HD is filling up really quick even though I'm not downloading anything...