Huge data usage overnight

I awoke this morning to at least 25 text messages from AT&T stating that I had exceeding my data plan and was being allocated extra GBs for their customary fee. I was nowhere close to my data plan liimit when I went to bed. I lookd at my AT&T account and it showed that I used 30gb of data between 3am and 6am. How could this be? Besides the fact that I was asleep, there is/was nothing different about my data usage during my hours awake. I went to AT@T in Waco, TX and they told me to contact Apple. I called Apple and they told me that it was an AT&T issue. I have a data overage bill of approx $400 that seemingly nobody can help me with. Any thoughts?