I can't help but think that that's a wrong reading. I've got my CPU reading at -16C before (yes, thats at below freezing with aircooling), and I can get that reading out again anytime....
I wouldn't trust it. Unless of course your ambient (or room temp) is at 10C or something. Your CPU temp reading can NEVER go below room temp even with watercooling (you might get 1C below due to calibration problems).
Either way, if you're not crashing in the middle of something, temp is probably not a worry for you.