HELP!

I have been an RN for 3 years but just recently took a job at a LTC for the first time. Well, an issue occurred the other night and I would appreciate some advice from ppl that have worked LTC longer than me. I came on @ 11p. The nurse leaving had worked 7p-11p (lets call her nurse B). I got there, got report and was handed the keys so that i could open the med cart to count narcotics. Upon counting narcotics, i noticed a discrepancy. Seeing as how neither I nor nurse B had ever been in this situation, we called our nurse hotline (which is either the unit manager or DON). I asked the unit manager what I was supposed to do in this situation and she informed me of what to do and that she would investigate the following day. well, i got a call from the DON the next am and she stated that I and nurse B share equal liability because i "should not have taken the keys". I did everything that i was told to do by the unit manager. She was aware and wanted to wait until the next am to investigate. i dont understand how i share any liability in this. can somebody please help me?