To give you an idea what to look for. Goto Application Manager, find and click on Google App then Data Usage. Scroll down where you should find the option to Restrict Background Data.
Many other apps have this option and may be using up your data even if you never clicked on them before.
You may also have check it again after any updates.
I'm fairly certain too there is option which can restrict background data on all apps. with just one swipe.
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