Every time my daughter has pulled on her ears, it was because she had an ear infection. Now she is old enough to say her ears hurt and then I can have her checked, but back then, when she could not speak, tugging on her ears was her way of showing me she was in pain, sort of how some children point at their throat when they have a throatache because they are too young to say "my throat hurts". She has done this when she has a cold and her ears popped when swallowing, as well as when her ears were newly pierced. Have the doctors tested to see if she has a cold? Does your granddaughter wear earrings? Maybe she is allergic to something, if not metal in earrings, maybe a sheet or pillowcase on which she lays her head down. Better safe than sorry, take her to the doctor for a checkup, to rule out any health problems or maybe consider getting a second opinion. A doctor should not be so quick to dismiss something like this unless they have run all tests necessary to rule out a problem or can offer an explanation for the tugging, so I would consider a second opinion if you're not getting any answers.