A family of Microsoft word processing software products for creating web, email, and print documents.
It's possible your document has acquired some form of corruption.
Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it.
If that doesn't resolve the issue and you don't have any endnotes in the document, you may be able to repair the sequencing by temporarily converting the footnotes to endnotes, then running the macro here: http://www.msofficeforums.com/word-vba/16610-repair-two-interspersed-endnote-numbering-schemes.html