Just some educational notes...
When you install new software on your computer, regardless of where you install it some programs still install files to the C: drive so the software can function properly. This of course isn't always the case, but for a lot of the larger software it is.
For instance, you can install Photoshop to a different drive, but Adobe installs Common Files as well as some DLL files on the C: drive. This happens with some games as well. Take Sims 3 for instance, you can install it to whichever driver
you want, but it will create a folder on your C: drive as well.
Windows Updates will definitely take up space as you update more and more. To be perfectly honest with you, going from 18GB months ago, to 14.9GB now is nowhere near horrible. I wouldn't be hugely concerned until you notice a large decrease in a very short amount of time. In the mean time, as has already been suggested just run routine cleans and keep an eye on what's being installed.