Deeds don't get transferred or assigned. Title gets transferred. If the prior owner/borrower gave a deed in lieu of foreclosure to the mortgagee, then the mortgagee became the owner. Subject to the specifics of the deed in lieu, that may have effectively extinguished the mortgage, which might make the assignment of the mortgage a nullity.
It would be helpful if you provided a timeline:
mm/yyyy - mortgage executed.
mm/yyyy - owner gives mortgagee a deed in lieu
mm/yyyy - deed in lieu filed/recorded with county clerk/recorder
mm/yyyy - mortgagee assigns mortgage to you
Please also include any other relevant events in the timeline and also answer the following questions:
1. For what reason did the mortgagee assign the mortgage to you?
2. Did you give consideration for the assignment? If so, what was the consideration?
3. At any time, were you represented by an attorney in connection with this?
4. Who drafted the assignment of the mortgage?