inserted links default to wp-admin folder

I just upgraded to version 2.5 and, after scratching my head, I’ve found that when I insert a link to another page on my site, it seems to default to the wp-admin folder when I publish it. I use the default permalink structure (if that’s related).

For example, if I needed to make a link to a page, I would type this into the insert link window:
index.php?page_id=275

and it worked just fine until I upgraded to version 2.5. After doing that, if I type the same think into the insert link window, it is transformed to the following after saving or publishing the post:
https://www.isaacandnathan.com/wp-admin/index.php?page_id=275

Which causes the admin login page or the dashboard to display instead of the desired page or post.

Am I doing something wrong or is there a setting I need to change? I really don’t want to have to type my domain each time I make a link, and I didn’t have to before I upgraded. Can someone help?