After an auto accident, I have had more pain than I had prior to the accident. I believe the accident aggravated it, or increased the injury. Is the other person responsible?
The other person is responsible for all damages that they cause. Because you had a pre-existing problem, they are still responsible for the injury that you received after the accident. This is called the egg-shell plaintiff rule. The defendant is responsible for all injuries that they cause.
As an aside, sometimes your doctor will tell you that the other person is not responsible for this. Just as I do not practice medicine, your doctor should not practice law. If you have an increase in pain or other symptoms, talk to an attorney about your rights.