sudden data usage spike

Over the last two days, my iphone has burned through 3 gigs of data, despite my being mostly at home and without doing anything out of the ordinary when I'm out running errands in the car. I looked at the data usage in settings but I couldn't pin down what was doing the damage. For the present I have data switched off, and I reset the stats. The phone is iphone 7 running 14.0.1 Any ideas?