What does it mean when you see a dead person in your dream?

Let me start from the beginning, its my great grandmother. She was someone I loved more than anything in the world. She practically raised me, I used to see her every day of my life. She was the greatest person I have ever known, and probably ever will know. She was a truly beautiful person on the inside and out. She was an angel on earth. She got sick in early 2012 and eventually suffered a stroke leaving her unable to walk or talk. The last time I saw her alive was the day before she died. I held her hand as she lay dying in her bed, She smiled the biggest and most beautiful smile, and I leaned in and gave her a kiss and told her that I loved her for the last time. I wished so much that I could have heard her voice just one more time. She died early that next morning on June 29, 2012. About 3 or 4 days later I had a dream that I was just waking up. I had gotten out of my bed and it was the most beautiful morning I have ever seen. I could hear the birds chirping and I could see the sun streaming in through the windows. I walked out of my bedroom and just as I got to the end of the hall and rounded the corner to the living room, there she was. She was sitting on the couch wearing her favorite clothes and the shoes I gave her for Christmas. The sun from the living room window encased her entire body, it was so bright and so beautiful. She looked right at me and smiled that same beautiful smile. She didn't speak, I tried to say something but nothing came out of my mouth. Then I woke up and that was the last time I saw her until now. Her birthday was just a few days ago, and I was thinking about her, and I even wished her a Happy Birthday in my prayers. But last night in my dreams I saw her again. I don't quite remember what my dream was about, It was very random, all I can clearly remember was seeing her. I was walking in a crowded place and I turned around to see her standing behind me. She just stood there and smiled that same beautiful old smile. She didn't say anything, but this time I could say something. I told her the words "I love you" exactly, except they were in a whisper for some reason. I could not speak in my normal tone. She did not respond. She just stood there and smiled, then she started to fade away and I woke up. I thought it was very strange since I did not see her last year on her birthday, or this year on the day she died. I've only seen her those two times. The first time I could see as a final goodbye, but I don't understand seeing her again for no apparent reason. My life is pretty well I guess. Not too bad, I'm making it. Nothing new is really going on, things are just a little slow. Maybe it would be easier if I were in some kind of trouble but I'm not. Can someone tell me why I saw her again? Is it some sort of sign?