I agree with the last responder, you're not a bad person for not grieving. When someone lives a long, happy life and dies comfortably, it's understandable that their death (rightly) seems like, just the natural order. An elderly family member of mine died that way, she had been healthy her whole life into her 80s, then one day she wasn't feeling well so she went to lie down and died peacefully in her sleep, in the home she raised her children in and still shared with her husband. I can't think of a better way to go than that.
On the other hand, when my grandfather died when i was 17, my mother was so angry and grief stricken, all my energy went into supporting her that I couldn't feel anything myself for a long time. I think it was about a year before I even cried because it didn't seem like there was room for my grief along with hers. Sometimes it's also a case of shock; the first stage of grief is denial. It's normal to feel nothing for that reason then suddenly start to feel sad, depressed, angry etc.
In sum, i think the best thing would be to go easy on yourself. Your current feelings, whether they change or stay the same, definitely do not mean that you didn't love her. Grief is an incredibly unique process and there's no wrong way to feel.