Costco Run?
Did ya loan it to Kramer for a food run??
Sorry, had to say it. Seriously though, if it just started smelling bad, and you've had it for a while and you've never smelled this before, I'd say check everywhere. Make sure no wild thing crawled up into something. Or if maybe you ran over a dead thing, and um, now some of it is attached to your undercarriage....ewwww.