Rest assured you didn't use 100GB of mobile data over night
CyanogenMod pushed out a small update to CM10.1 yesterday bumping the software up to RC3, but it turns out there was a small math calculation bug that gave users quite a scare the next day. Because of changes made to the way the software handled complex math calculations, many devices had issues anywhere the system had to calculate and display numbers. The end result were some shocking data usage counters, which often showed devices using 100GB or more over night from very basic usage. The bug, it turns out, was giving exponentially (yikes!) incorrect results when counting on any device with a chipset other than those the new code was tested on, which understandably caused some panic.
Fear not, your phone didn't randomly pull down over 100GB of mobile data while you were sleeping last night -- and the update today to CM10.1 RC4 has fixed the issue. Things like this happen, and that's part of the nature of flashing custom ROMs. Luckily the issues were simply cosmetic, and can be fixed with a quick update to RC4 from the get.cm source link below.