I'm kinda at a loss Pollux, not sure what to say. I didn't realize this temp change occurred in just one day. Had thought maybe it was gradual over time, and you had just noticed it.
If I were to trouble shoot the same problem on my pc, I'd first look for any physical/mechanical problems or recent changes; such as the pc being moved, jarred on accident, slow fan going out (though they usually make noise), or possibly unnoticed temperature increase in the room (can have a very significant affect.)
As far as the bios goes, since you haven't changed it and it worked before, I doubt that's the cause of the increased temperatures you're seeing. My general rule of thumb with the bios is to always keep it updated to the latest (and known stable) version. Then again, many prefer to stick to what ever version they know works works well, and never change. Probably wouldn't hurt to upgrade, but from the given information, it doesn't seem to me to be the source of the problem.
I hope it's something as simple as an unoticed increase in ambient room temp, failing fan, or HS that needs to be re-seated. If there's no other problems with the system, stability issues and such, there's probably not much to worry about. However I can understand the frustration. It would be driving me nuts to figure out what caused the sudden change...wish I had some better advice for yas.
I feel for ya man...Hope you get er' figured out.