Details on Android OS massive data usage Yesterday Android OS on my Nexus 5 transferred 44,7 GB (!) data over both WiFi (42,2 GB) and mobile data (2,5 GB). I have no idea why the data transfer started, and I have no idea why it stopped after one day, and I have no idea what data was transferred, and to/from where.

I haven't changed or installed anything on the phone.

Is it possible to see exactly what data has been exchanged by the Android OS and why?
Yes, you can - details here: http://www.makeuseof.com/tag/why-you-dont-need-third-party-apps-to-manage-background-data-use-in-android-jelly-bean/

From the article: 


  "To access the Data Usage screen, open your Android phone’s Settings
  screen, and tap Data usage under Wireless &amp; Networks. You’ll see a
  graph displaying how much mobile data all apps on your phone used over
  a period of time. Specify the “Data usage cycle”  time period that
  your carrier uses to determine your monthly data usage and you should
  see the amount of data you’ve used in the past billing cycle."


Hope it helps!