Unfortunately, as an operator, we’re unable to see which precise app or software is linked to this, since these data are only registered in your device. Most devices allow you to check this. In Android devices, for example, you can find this in Settings > Data usage. In Windows Phone, it’s located in Settings > Data Sense, and on iOS in Settings > Mobile network. On an iPhone, also make sure to check if the feature “Wifi assistance” is deactivated. You’ll find this too in Settings > Mobile network, all the way down at the bottom. If this is activated, your device will automatically switch to mobile data if the Wifi signal is too weak or drops. And since a data session can remain active for several days, I’d like to recommend you to switch this off entirely when you don’t need mobile data (overnight, for example). This will prevent your device from continuing to use data via mobile internet. To abort all active data sessions, it’s advised you reboot your device.
Also bear in mind to manually close app when you no longer use them. I’d strongly advise you to also check if the option “Update via mobile network” has been deactivated (for both apps and software updates). Moreover, it’s recommended to download a specialised app to check which ones of your apps are the most data-consuming.
These few tips should help you control your data usage more.