Problem with NextPage and Permalinks

Hi, I recently changed my permalinks structure to set them up so they look like this:

http://thepete.com/post-title/

The thing is, when I use a nextpage tag, the page links look like this:

http://thepete.com/post-title1/

and

http://thepete.com/post-title2/

These links to point to files that exist–however, if I manually change the link to:

http://thepete.com/post-title/2

I will then get page 2 of my post. Can anyone please tell me which file I need to tweak to fix this, please? I’m sure I can fix it if I just know which file creates those links and roughly what the code looks like. Any help would be greatly appreciated. Thanks!