Mortgage Sold and I'm Behind on Payments

Your past due payments are still past due, and they will remain thus and be accounted for accordingly until either the debt holder agrees to some form of alternative plan for you (i.e., debt restructure) or you bring the payments current.

It is important to remember that when a mortgage debt goes into default for an extended length of time, the debt holder (often referred to as the investor) may elect to liquidate the debt in order to limit the loss related to this specific debt that is carried on the books.  Liquidating the debt caps the loss or puts a finite or fixed amount to the loss on the books, preventing it from becoming larger. 

When third parties purchase defaulted debt, many times servicing accounting errors can occur.  You have remedies available to you if you suspect this may have happened.  Which remedies will depend on the nature of your mortgage loan (i.e., first or second lien, primary residence or secondary / investment). 

But the bottom line is that your past due amounts are still past due, and delinquent interest will continue to accrue on the account.