I am sorry to hear about your grandmother. I actually had a similar response to my grandmother's passing, and felt strange about it. I think for me she had been declining for so long and was in a lot of pain, and I had been saying goodbye to her slowly over time, so the actual passing did not make me as sad as I would have expected. She was ready to go. It was more of a situation where I knew I would miss her, and I will sometimes think of her fondly and wish we had more time together, then I might get a bit sad.