I am an AT&T user and have not seen any jump in cellular data usage. I would suspect some sort of app issue myself. Maybe one of those "social" apps (they all seem to want that hot button tag) that lets your friends know where you are at all times and lets you see where they are. Or a navigation app that was poorly written. Take a look at location services, think back to what got update on your app suite in the time frame things went bad.
Also, there is a cellular data counter in the General settings app for the phone. Reset that baby at the start of the day and check it thru out the day as you do things. Might give you a clue as to what is being used, and when it is being used.
I guess I will be real surprised if this does not turn out to be some poorly written app that is just chewing up data like crazy.