If you are hurt, you must be seen by a doctor.
My standard advice is: if you think you're hurt, you should absolutely be seen by a doctor. I've had countless people tell me over the years that the full extent of their symptoms didn't manifest for one, two, and sometimes three days after an accident. Particularly in a rear-end accident, it is not at all unusual for someone not to notice symptoms or to have problems until the following day, even sometimes two days thereafter. If you feel pain in any part of your body, if you're not able to do things that you were able to do before the accident, you should absolutely get yourself checked out by a medical professional.My advice here is not to wait. Don't give the insurance company a chance to say that person wasn't hurt because they weren't getting medical attention promptly.