What Paul was saying is that you have your headers/footers set for different even/odd.

I am guessing that you are using the Page Number button on the ribbon and picking a location (top, bottom, or side) to insert your page number. This replaces any existing header (or footer).

Many people want page numbers to be on the outside (unbound side). Go into the header/footer without the page number showing. If necessary tab to where you want it displayed. Press Alt+Shift+P to insert a page number field. Alternatively, when your insertion point is where you want the page number, use the page number button to insert a page number at the current location (not top, bottom, or margin).

Page Numbers in Word 2007-2013
Header/Footer Settings Recap