Why has my Samsung Galaxy S2 become a data hog?
After reading the comments, there's not much else to suggest for you to check on your phone.
So, here's two free APPs to help you out:
My Data Manager
Main features:
  • Simple to setup and simple to use.
  • Know what apps are using data with automatic monitoring of your app data usage.
  • Set usage alarms to avoid overage charges and “bill shock”.
  • Track historical app time and app usage using easy to understand visualizations.
  • Ensure you are using your ideal data plan and maximize how you use your data.
  • Save money by using your plans more effectively.


3G Watchdog
Main features:
  • monitors your Mobile Internet (4G/3G/Edge/GPRS) data usage.
  • shows a status icon (green/orange/red) in the notification bar.
  • has a detailed report page.