First let me say you have my sympathy for your loss. Next let me try to explain something to you that you may not have considered. Traditional religion believes that people die and go to heaven or hell. But we are finding out more and more that this is not all that takes place. People who work in this field, in attempts to understand what does happen to someone when they die, are becoming more and more convinced that death is nothing more than a transition from one dimension to another. The soul or spirit within simply leaves the physical body and moves to a new dimension of life. Because that dimension has no physical boundaries the spirit of someone can move back and forth between our physical world to their spiritual world. When a person dies their love and concerns for family members, friends, etc. does not leave them - it stays with them - and often times what people term as ghosts is nothing more than the deceased coming back to check on those they love. It is their way of communicating with those of us in the physical form. 

Now, as for the question, why you? Probably because you have more natural psychic abilities than others around you. No, you are not going crazy you are just being contacted. You need to listen carefully when these events take place...clear your mind and let your mother communicate with you in thought. Perhaps there is something special she wants to tell you or have you pass on to some other family member. Or perhaps she is just trying to let you know she loves you and will be looking over you from her spiritual world. This is nothing to be afraid of ... in fact when the pain of loss wells up inside it is a great comfort to know the one we love is just a dimension away and can still communicate with us in some form or another.

Years ago I lost my father - I experienced much the same of what you are going through. I was 17 when he died. I can remember smelling things in the house that were only tied to him. I remember hearing the click of his lighter in the middle of the night, even though his lighter lay in a drawer. I remember hearing his voice when I drove his car, warning me of various things on the road. I took comfort in knowing, that inspite of death of the physical, my father was still with me in spirit. I suggest you do the same with your mother - know she loved you and her family enough to make contact with you from her new world.