Word doc populates file with text from an unrelated Word doc

Has happened on two different occasions, weeks apart. Running Word for Mac 15.34 on OS 10.11.6, (newest OS my old MacBook Pro allows). I create a doc, save it as usual. Upon reopening, my new text is gone and the document contains the contents of an older, unrelated Word document. Worse, if I am unaware the text has been replaced, and I attach the weird doc to an email, as I did yesterday, it's empty--zero bytes. Zero, even though, on my desktop, it shows as, say, 84 KB and contains the unrelated text. It's been hell to lose m work this way. Anyone?