Is it normal to have nightmares in which my deceased grandmother is not herself

My grandmother died in February 2011 and for a year and a half afterwards I had dreams (nightmares really) about her. In most of the dreams she was not herself; it was almost as if she was possessed or simply a dead being walking around. She would never recognize me with the happiness that she actually did when she was alive. She had a dead, darkness about her and the only resemblance she possessed of my grandmother was her physical body. The dreams stopped after about a year and a half, and I thought they were gone for good, but just the other night I had another one. I'm curious as to why I would have had such a nightmare about her considering I've felt very at peace with her (and I believe she sends me butterflies as signs, quite often too when I least expect it) but I'm more interested in finding out why it is that I see her in such a disturbing way in my dreams.