Ok, I think I've identified the fact that it's only happening on this current file I'm writing. This was a case in which I'm writing a second edition to one of my older books. I had long lost the original text file, so I scanned in the pages of the book (only 48 of them) and saved them as "editable text." I went through and cleaned up the occasional mistake it made from scanning and cleared the formatting to start from scratch. Then I started editing/adding new text and formatting as I went. It was some time during that process that the problem started showing up.

Thinking that this might have something to do with it (the fact that it was scanned text), I copied the entire document and pasted it into a new document using the "paste special" function as "unformatted text." I went through and started formatting this new document, but now the problem has surfaced again still.

So I don't know what to do. I'm now going to try to send it to my wife's computer to see if it does it on her's too. I think she's running Office 2013.