Have you updated to iOS9? If so, you may want to read this article to avoid unnecessary data overages.
With the update to Apple's latest mobile iOS version, iOS9, there is a new "feature" called Wi-Fi Assist that is enabled by default that will automatically change your connection to cellular data when your wifi connection is low.
What this means is that you may be unknowingly using data that you were unaware of causing your data usage to skyrocket and ultimately if you don't have an unlimited data package see your pocketbook skyrocketing as well.
To turn this feature off go to Settings>Cellular>Scroll to the bottom and toggle or slide the button next to Wi-Fi Assist or see the following webpage.