I did not change any setting on my note 3 after upgrading to Lollipop. I disabled any data updates especially on the google store but it just keep on insanely using my mobile data daily.
Hello. You say you didn't change anything in Settings, and changed apps to wifi only. You should be able to see what else is eating mobile data by scrolling down the Data Usage screen on the mobile data tab (not wifi).
Also, perhaps obviously, check the Sync Settings in the Google and Samsung Accounts.
I think any major update resets all sync preferences, so they could have changed without your input.
Any services you had disabled may no longer be.
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