The cause is most likely due to background data being used by apps. You can restrict background data by going to settings> data usage> and I think there is a restrict background data option near the bottom.
Since apps updating in the background use probably around the most data, you can set your applications to update on wifi only.
Open the Google Play Store Google Play Store app.
Touch the Google Play Store Play Store icon > Settings.
Under "General," select Auto-update apps.
Select Auto-update apps over Wi-Fi only.