Unexplained Large AT&T Data Usage

My wife and I have our iPhone 5's (iOS 6.1.4) set up exactly the same way. We use AT&T LTE Data when not connected to WiFi. Recently her data usage has gone wild, with no explanation, when she is not in a WiFi atmosphere at home. She uses LTE Data far less than I do, yet we have excess charges for over 800 MB this month on her 200 MB plan. This has never happened on my phone, and just began a couple of weeks ago on hers. She uses her phone for data much less than I do. Is there something I may be overlooking that causes this? I am going to visit an AT&T store tomorrow to see if they can explain it. However, I am more optimistic about getting good advice on this forum. Would appreciate any help.