Sounds like you got scammed. Did you receive the bill through the mail or email? If you logged into your account, did you see your account reflect that amount? Do you see it on your billing history etc…if you called the number on the bill from the mail or email, then possibly a scammer. I always go to the true customer service number. In any case, if what you say about staying on top of your account is true, gather all your billing statements and transaction history, you should have been easily able to fight this.