Are you using iOS 9? Are you suddenly getting messages from your phone provider that you’re using up your data at an alarmingly high rate?
The problem might well be in a new feature found in iOS 9 called “WiFi Assist”. What this item does is whenever you’re using WiFi and that signal becomes weak, this unasked for handy dandy “WiFi Assist” automatically switches over to your cellular carrier to “boost” the signal and therefore your reception. Of course, this is at the cost of your monthly data quota.
I happened to notice this the past couple of months even though I don’t use the internet all that much on my phone and had never reached my data limits before.
Anyway, for more information, including how to turn off this annoying “assistant” no one asked for, check out the article below: