GUID in database is not unique

Ever since I changed the URL of my site, when creating new posts, the GUID stored in the database (and subsequently outputted into the feeds) is NOT unique. In fact, it is just the base URL of my site (http://mysite.com) — there is nothing else unique after that.

This is causing a problem with my feeds, since the feed readers are not recognizing new posts since the GUID is the same for a number of different posts.\

I know some people were having this issue before 3.1 was released, but I am using the latest version (3.8) and still having this issue.

Any ideas on how I can fix this? I know I can go into the database and manually change the GUID for some of the posts, but when I create new posts it will still just be the site URL.