On your iPhone, go to Settings/Cellular. There is a list there of apps. The numbers under the apps is the amount of data that app is using on the cellular network. Which app is using the data? This is the first step in understanding what is going on. Let us know which apps are reporting data use.
You can also go to the bottom of the Cellular list and "reset statistics". Keep an eye on that and it can tell you how much data you're using. Make sure your wifi is strong. Reboot your modem/router.
The iPhone 5 models (and possibly the 6 too) consume cellular data whether or not you are on a wifi, until you manually set it not to. If you are like me and live in a weak LTE coverage area the phone will consume small bits of data 24/7 just trying to acquire a tower signal.
Over the months I have been able to throttle back my phones data consumption by turning the LTE function OFF. This overall has been the biggest difference maker, I had found. With LTE turned on, I was getting rampant, round the clock data usage.
Also the latest iOS has multiple ways in which it uses location services, not just the apps, but the phone itself. You will most likely never even notice it... until your monthly billing statement punches you in the face.
Settings => Privacy => Location Services => scroll down to System services => scroll down to Frequent Locations => Clear them and turn this function OFF. The factory default is set to monitor and track your where-abouts, and yes it uses cellular data to accomplish this. I do not know if the 6 models do the same thing.
I posted this video almost 2 years ago. I have since witched to a Sony Xperia / Android, but the data presented here is still valid.