Why Does My Head Pain On Throwing It Back?

hi im a female of 21 yrs old.its been more then a month when i throw ma head back i feeli in da middle of neck. But when im standing in normal postion n when i move i head i dnt feel anytyme.If I touch da back of my neck i dont feel pain. i had my Xrays done they were normal i have taken muscle relaxants but it makes no difference. i only feel pain when i throw ma head backwards