The visa office may extend your medicals by an additional year within 15 months of your original medical examination date. See http://www.cic.gc.ca/english/resources/manuals/bulletins/2012/ob423.asp
Note that it is the VISA OFFICE that requests this extension - there is nothing you can do about it. If the visa office requires updated medicals, they will advise you. You can choose at that point to have them done again, to withdraw your application, or to fail to respond in which case they will terminate your application.
Good luck!