Over billed

I have unlimited data for 2 phones. It started on the 8th of September. Now att is saying I owe over a $1000 dollars for what amounts to 18 days (from August 20th to September 8th). They called me several times.when I returned there calls they informed me that I owed nothing. Then I get a disconnection notice in the mail. I called them back, now theyou say I have an over due bill. They also said it is due to data usage overrun. I'm trying to get a breakdown of they usage for that period but I cannot find it. What is going on here!!!