What would cause a 2 year old to suddenly scream in pain and grab the back of her head on the left side?

my son(2 yr old) had the same thing it started suddenly,and was so painfull he refused to walk around , and stayed as a stiff neck for about 3 or more days going away little by little. we took him to the emergency room and since there were not any other symptoms they released him after 24 hrs and diagnosed it as torticolis which is about the same as stiff neck and could show up suddenly