As mentioned, your phone will give you a breakdown of data usage, however, your problem is not uncommon with iPhones.
These is a bug that either uses or misrepresents data and has been an issue across all networks worldwide for the past 2 or more years.
It's random and doesn't effect everyone but there is no logic to it when it does hit you.
Best way to avoid it is to get a non apple phone.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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