If I am receiving a bill for something that was discharged in a Chapter 7 bankruptcy, do I have to pay it?

The question you pose is fact dependent.  Was the debt with the second mortgage company reaffirmed in the bankruptcy proceeding?  If so, you would still have liability.  Was the creditor listed on schedule D of your petition as a secured creditor?  If not, the debt may survive the bankruptcy as they were not put on notice.