Wi-Fi Assist is a cool little feature in iOS 9 where your phone switches over to your cellular network when your Wi-Fi connection is crappy. Unfortunately, some users, including Mike Murphy over on Quartz, are reporting that it's eating through their data plan.
Wi-Fi Assist essentially turns off Wi-Fi and switches over to cellular when your connection is bad. This tends to be when you're on the fringes of your Wi-Fi network, like if you're standing outside your home or office. Do this enough, and you'll start to see it in your data consumption. To check your usage by app, go to Settings > Cellular to see your data consumption (you can also head to your cell providers web site for your overall usage). If it's up a lot, you can disable Wi-Fi Assist by just scrolling to the bottom of the page.