Can brain damage from a blow to the head that knocked me out not show up on an mri. headache vomiting dizzy forgetful angry times 5w mri negative. ?

Post-concussion : syndrome can cause the physical, mental & behavioral symptoms you describe after a mild brain injury that causes no visible or clinical signs of damage to intracranial structures. . PCS starts within a week-10 days after a concussion & can last up to a year. See http://www.mayoclinic.org/diseases-conditions/post-concussion-syndrome/basics/definition/con-20032705