First, you should check your bankrutpcy schedules to make sure that the creditor sending you the bill received notice in your bankruptcy. Secured Creditors will be listed on Schedule D and Unsecured Creditors will be listed on Schedule F.
If the creditor appears, then it received notice of your bankruptcy when you filed. If you received a paper bill, then call the company to tell it the date you filed your bankruptcy case and the case number.
If the creditor does not appear, then the Schedules need to be amended to include the new creditor. If your discharge has already been entered, then an adversary proceeding may be necessary to discharge the debt.
It is very important to make sure all your debts and creditors are properly listed in your bankruptcy schedules to ensure that the bankruptcy is able to discharge your debts.