Rooted lol.

Anway TS, it happened to me too a few weeks back. I found the cause to be the software miraculously update on its own while on data and the wifi turned off.

Try to double check under the Cellular tab, theres a section called 'Use cellular for' and untick the unwanted apps and I believe you are able to see which app is consuming the most data under the app name in the same tab.

If it's still the same, reboot your phone and see if there is any changes.

Good luck.