Hi thanks for your question.
The golf ball size knot which you had developed on top of your head following trauma to your head.Most probably was a haematoma
(collection of blood under skin and muscle) which got resolved of itself leaving behind a scar what you call as a stright line.Scar tissue usually dont't contain any hair follicle that is why it is bald.There is no harm in a Doctor consultation as if the scar is large one, Doctor may advice you removal of scar surgically.Small scars
usually disappear of itself without any treatment.
Hope this answers your question.