What Causes Severe Body Tremors And Muscle Pain Post Surgery For Anal Fissure?

Hello Doctor,my sister(28) had an operation for anal fissure.After discharged from hospital,the same night she suffered from extreme body shiver and trembling with leg muscle pain and headache.Its been 8days and 3 times of that same case.we went to the hospital for follow up,and asked the doctor,what causes that shiver.Doctor told us,its not the medicine may be you should go to neurologist.Honeltly speaking,Im in dilemma,please help