iPhone 6, iOS 8.3: Data is being used when cellular data is turned off. Why?

Since I updated to iOS 8.3 my cellular data has consumed my entire data plan even when I turned cellular data off in the "Cellular" menu option in settings. Why might this be happening. It wasn't happening before I updated recently and I never change any of my features before it started. I have turned off all back ground data, and application permission to use data and still it is being used. I think it might be iMessage but I'm not sure why it or anything would being using data after it was disabled.