I killed it.
Yesterday I installed 2 addons. The first was the About Us page, and the second was the 3 in 1 Captcha addon here.

The About Us page seemed to work fine.
I backed up all the files (but not the db) before installing Captcha. After I installed the Captcha, I couldn't get it to validate, so I uninstalled it, deleting the old files, and reuploading my backup. I ran the uninstall.sql to finish the installation.

Now, whenever I try to check out, or checkout using paypal, I get the whoops! Your session has expired. Any suggestions as to what may have gone wrong?