I am not sure what this could represent. One thought is that it may be a traumatic neuroma (balling up of a nerve after injury) however I have never heard of this being on the scalp before. You should see your doctor, to follow-up on this. Depending on what the doctor finds, plain X-rays could be done to see if this bump is made of bone. Otherwise a MRI can be done to better visualize the area. Good luck.