rocking

my daughter has been rocking on and off since she was 18 months it started after having my second daughter it was quite bad but i thought she had grown out of it she no longer does it at home at all but in the last couple of months it has been brought to my attention she is doing it all the time at school. she goes off into her own little world and gets really hot and sweaty when she does it the school has asked me to sort it out have tried the doctor but said it is probably a comfort thing dont know what else to do have tried rewards have tried sitting her on a pillow have tried talking to her about it but says its a feeling that wont go away unless she rocks please help am extermely worried thank you