A car crash is a stressful event, and when stressful things happen, your body produces adrenaline and endorphins. This reaction can mask the immediate appearance of even serious injuries.
Also, the body heals by rebuilding tissue and attacking the infection. This response can cause swelling and inflammation that doesn’t begin until sometime after an injury occurs. There’s also a process of emotional healing after an accident. You might not realize how much you’re hurting mentally until long after your crash.