Settings>General>Usage
You can see which ones might be data hogs. Also, check your location services to see if one of your apps is using it more often. It will have a puple location icon next to it if it is currently active. gray if it's been used recently. Scoll all the way to the bottom of Location Services and look at System Services. I have all of mine turned off except traffic. Make sure you are not sending data to Apple unless you want to. Settings>General>About>Diagnostics and Usage. Also turn on "Limit Ad Tracking"