It has been all over the news. I checked my Iphone and had this feature turned on!!
Have you downloaded the IOS9? If you have then you could be getting charged for this extra feature or it could be eating your data. The feature is supposed to automatically change your phone to 4G if you lose wifi BUT you are being charged for the privilege.
To switch this OFF
Go to Settings>Mobile Data (or Cellular Data if in the us)
Scroll right down to the bottom to Wi-Fi Assist
Switch it off
Hopefully that will be the end to it.