A new iOS 10 feature may be causing you to use more cellular data than normal, even when connected to WiFi. With iOS 10, Apple included a new feature called Wi-Fi Assist. Basically, anytime your iPhone is connected to Wi-Fi and the signal on the wifi connection is not strong it will supplement that connection using your cellular data. This can cause you to unknowingly use more data than you may have intended.
If you’re like me, anytime I connect to Wi-Fi I don’t want to use my cellular data. To disable this, simply go to Settings>Cellular, scroll all the way to the bottom of the screen and turn off Wi-Fi Assist