I've recently started getting this message too.
My D drive reportedly has less than 500 Mb free.
No I have not used 'save as' recently, in fact I've hardly saved anything. I had a rather full waste bin, which I've emptied, I've also deleted several old FTM trees & back-up files but the notice still pops up now and again.
I suspect malware in which case I'm scrapping Windows and going to Linux.
That's a bit of a dramatic step.
What is your setup? Which version of Windows are you running? Do you have a C: drive?
What is your D: drive - is it a stand-alone drive or is a partition of another drive? How big is it that partition? How big is that drive? What do you have installed on it? Do you backup your documents, pictures, music and videos to that drive? How quickly is the space diminishing from that drive?
How much space do you expect to have?
Have you run any software to see if you do have malware?
If you do want to switch to Linux, will all your software still run? Do you have backups of everything important?