There is a condition known as 'stress-induced analgesia' that sometimes occurs following the scare of an accident and enables an injured person to temporarily block out the pain mentally. When a person is in a very stressful situation, the body releases noradrenaline into the bloodstream. Noradrenaline a/k/a norepinephrine is a hormone that acts as a neurotransmitter in the central nervous system and sympathetic nervous system that blocks the transmission of pain to the brain. When a person is 'stressed-out' by an accident, they may block out the pain until a later point at which things calm down, the noradrenaline levels return to normal as suddenly the pain is perceived by the brain again.