Install
My Data Manager app.
It will tell you what app is sucking all your bandwidth.
It shows for 3G, roaming and WiFi.
I recently had something like that happen on my unrooted phone. Turns out it was Amazon app store redownloading itself each time there was a notification and Amazon MP3 app downloading prev/next and current song when I was streaming music. So I no longer stream music but download it instead then play the music. I don't need all those extra songs on my phone taking up space in the cache.