Most likely problem on your side is a rogue app or malware.
If you really don’t think you are knowingly or even unknowingly using your data, ask them to start logging your data transfer and send you reports with the data rates, as well as the top 10 resources that you connect to in terms of data used.
When people have this problem, after they get reports back, they usually end up finding out one of their children was secretly connecting to their hotspot and watching youtube all day long. Best to get your carrier to provide a report. It will either put them on the spot, or it will show you where you have a problem.