Welcome to our community and thanks for posting. Sorry to hear your have used ore data this month than usual, we know this can cause concern especially when it's not happened before.
There may be a few factors that may have cause a surge in the use of your data. Do you run any background apps or that refresh or update your phone running? These can use quite a substantial amount of data, also things like auto updates.
If you're connected to WiFi we would also recommend turning off your mobile data as this will quietly run in the background and use mobile data if you lose the WiFi connection.
I have included a helpful link here on ways to save mobile data o your iPhone for future reference.