Look in Setting under Cellular and you should find a list of all apps that are allowed to use cellular data, and how much data they have used. You may find that you are the culprit... You can restrict any app you want to use wifi only for data. On this same screen you can reset all statistics so going forward you can track which apps you are using and the data usage.
Tom Wilds
Blythewood, SC
2016 Newmar Baystar Sport 3004
2015 Jeep Wrangler 2dr HT