On MS Word when I press 'save', it tries to 'save as'

Using MS Word v16.10 on OSX EL Capitan.

Working in a large document for over a year. Always saved fine. I usually press cmd S as I work. Suddenly every time I press it (or any other way of saving - drop down menu or onscreen) it brings up the 'save as' box and tries to get me to save the document on top of the previous one (replace it)

that doesnt work for how many times i press save - and it shouldnt be happening. Have tried restarting word and computer but it makes no difference. Any suggestions?