My wife had the same issue 5.2 GB in 3 days, normally it would take her 3 months to use that much data! After we got the text from AT&T that they were going to throttle here data, I backed up all apps, sms, sdcards and e-mail and did a factory, twice, formatting the internal card each time as well for good measure. Only reinstalled apps from the Play Store and the data useage went back to normal.
One thing you can try is install 3G watch dog pro, it ill watch what apps use data and how much they use it. I wish I could have installed it before I reset the phone because now I will never know what happened.
If you do install 3G watch dog pro and find what app it is, please let me know