So I have a mobile account with 2 degrees.

Over the past two months my data is being chewed through randomly. So much so that I had to get an add on pack to see me through last month. This has never happened before and usually I have loads of carry over data to spare. This months data has already run out 

For instance, the on-line data usage page reckons I used 675mb on Sunday night at 8:30pm (when the device was on WiFi).

I've checked shared data and its definitely the primary account device (iPhone 5c).

Any ideas why this would be happening? Rogue app perhaps?

What's the easiest way to narrow it down?