It doesn't look like your settings and/or Django environment are being set up correctly.
Are you running the dev server, or a production-grade setup like Apache/mod_wsgi?
Does your site work using the dev server?
Do the file paths in the long traceback look like they follow your virtualenv path? If not, I would assume that your virtualenv is not being introduced by the server process.
As to why this issue didn't show up until days later, it's possible that you are running a production server where things were working, you made a change, and forgot to restart the server process to pick up the changes (which broke things). The processes do restart on their own due to many memory times, so they might randomly notice this issue days down the road.
-James