Keep getting ALL media wiped off iphone when syncing

I don't even know the right language for this so bear with me. I had to wipe my iPhone 4S clean a couple of weeks ago (blank, down to manufacturer's settings) and put all of my apps back on. I have hardly any media on my phone as it is, I never go into iTunes if at all possible, etc. However, I have a couple of media things on my phone. For discussion's sake, I have an audiobook in Music on my phone, but not in iTunes, I can't figure out how to make that happen (I imported it from OverDrive Media Console directly onto the phone, if it matters) and I have the Hunger Games movie in Videos (both phone & iTunes). Tonight, when I tried to sync a ringtone from iTunes onto my phone, I got the message, "Music movies, TV shows, books, and tones synced to 'my iPhone'" from other iTunes libraries will be removed and items will need to be synced from this iTunes library."

I figured, "not a problem." I don't have any other libraries, I don't plug my phone into any other computers, etc. Well, it deleted everything (specifically the movie & the audiobook) but did import the ringtone. I was hoping that was a one time threat, that now that I've synced with *this* computer, *this* library, *this* phone (all the same as always, but iWhatever seems to think something isn't). It's not. I just tried to sync a song from iTunes to my phone & I got the same warning. As I was reloading the audiobook (through OverDrive), I tried to reload the movie (through iTunes) and I, of course, got that message again. I said okay & the book upload was, of course, obliterated.

What gives? How can I make it understand that this is my one and only library? And why is it saying that it will erase everything that didn't come from this library and then erasing things that came from that library?

TIA.