What Causes Left Sided Shoulder And Chest Pain Post Implantation Of Cardic Pacemaker?

My mother had a cardiac pacemaker implanted on 4/1/15. She has been having pain ever since. The pain is described as a stabbing pain into her chest and up into her left shoulder. She cannot lay on her left side. The cardiologist told her it is because she does not have a lot of body mass and it will just take time. She has a high pain tolerance but this is wearing her down. She is 74 years old and only takes thyroid medication. She is using icy hot cream, cold compresses and taking advil when necessary. Any suggestions?