Many car accident injuries can take hours, days, weeks, and in the case of several brain injuries, months, or years to become apparent.
Just because you felt fine at the scene of the accident, and told the officer that you didn't need an ambulance, doesn't mean you can't make an injury claim.
Most people will have a rush of adrenaline following an accident, keeping them from feeling even the most obvious injuries.
Many people also turn down the ambulance simply because they are worried about the costs involved.
As soon as you feel pain, get to a doctor so they can check you out.
Just because you turned down medical attention at the scene of the accident doesn't mean you can't be compensated for your injuries.