I just had the worst nightmare ever. It was a FULL repeat of seeing my father get shot. What could this mean?

My father got shot August 28th 2016. I don’t know why I would have a nightmare about it. It had all of the details perfectly. Seeing my father get shot, seeing him get shocked by the defibrillator at the scene twice, seeing his heart stop 3 times at the ER. The last time the doctors tried to revive him, it was too late. My father was dead. It was the worst day of my life. Now I had to see it all playback in my head,