Don't grieve alone; 13,000 members and growing
I don't think you should feel bad. When someone is ill for a long time, it's natural to start grieving quite a long time before that person actually dies. It's a kind of disengaging that protects you a little bit, and it is quite common in that sort of situation. Besides, you felt that way in part because you didn't want your mum to suffer anymore, and it makes sense that you would feel relief because now she is no longer suffering.
With that said -- grief affects each person differently, and often is even expressed in one person in different ways. So at some point(s) you may find yourself feeling very sad, even though you don't so much right now. Or you might not. Either way is normal.