Yes, that feeling is normal, especially because your mother was so significant in your life. The loss of a parent is the most common form of bereavement, and even as adults, we are seldom ready for the death of a mother or father. Regardless of our relationship with our parents, we will always be their children.
The death of a parent can bring many losses and changes. Who else will remember the first words you spoke or the way you used to sleep holding your teddy bear? There are other losses: the long-term friendship of adult child and parent, the helpful advice, the emotional support during difficult times, and the celebration together of birthdays and special events. The death of our parents often brings a realization that we have now become the older generation. A parent's death can be a keen reminder of our own mortality and a shock that often promotes a healthy reevaluation of one’s life and values.
For more information on grief and loss, visit: http://www.extension.org/pages/Loss_and_Grief.