THE DREAMSCHOOL RESPONSE
There are many types of dreams we can experience: precognitive, visitation, lucid. This appears to be a visitation dream because your mother is no longer alive in the physical world. 

It is possible for loved ones who have died come to us in our dreams because they have a message to communicate to those “left behind”.  Most often the purpose for this is to assure their loved ones that they are happy and at peace.  When the message is received by those still alive, there is an opportunity for them to release attachments, fears, worries, or sadness.  When we are able to make this shift and release them, it frees the soul of the deceased as well as allowing them to freely review the life they lived and everything they had learned.

When it comes to not being able to move, know that this is not an unusual experience.  It is called sleep paralysis and has been studied throughout the ages.  Sleep paralysis is a function the body does naturally when anyone is dreaming.  It describes a state of complete relaxation of the entire body which keeps the dreamer from acting out the dream. It is a natural protective instinct between mind and body.

When someone feels like they can’t move in a dream, they are in-between the stages of sleep and wakefulness.  The effects can be experienced in the dreamstate as if the dreamer cannot move.  This may be what you are experiencing, that is unless the “not moving” is connected to your fear and only you can answer that question.

HOW THE DREAM IS SIGNIFICANT TO THE DREAMER’S WAKING LIFE

To understand why this is recurring, ask yourself these questions.  What is the subject matter between you and your mother?  Is it the same conversation every time?  Is there something she says that causes you to be scared?  If not, what is making you scared?  Answering these questions is the first step in relieving the anxiety you are feeling.  The next step is to truly listen to what she is communicating to you.  When you are able to receive her message, the fear will likely stop. – interpreted by JRagan