Sometimes the original lender on a mortgage loan will "sell" the loan to another lender, which is called an assignment of a deed of trust.
This is very common - a bank will provide a mortgage loan to a property owner and then later transfer the loan to another bank in exchange for the cash value of the loan (a bank may do this, for example, if it needs to raise cash for some purpose).
The new bank then assumes the loan and receives the loan payments from the original borrower.
The property owner is notified of this and makes payments to the new bank. The payment amounts and other terms of the loan remain the same. The only thing that changes is the bank that is holding the mortgage loan.
To determine if a loan has been sold, check the recorder's index under the name of the lender or the borrower after the date of the original loan for an assignment of a deed of trust. This is the document in which the original lender transfers the deed of trust and the mortgage loan to another lender.