What to do if I got rear-ended and the woman that hit me is trying to contest it?

Assuming you have automobile insurance for the vehicle that you were driving that was struck from behind by a driver of another vehicle you need to contact your claims representative about the situation and take things from there. Your insurance representative will assign an investigator to the claim, have photographs taken of the cars and get statements taken.

It is possible that the driver of the other car will contest the incident. Your recourse is to be proactive by contacting your presumed auto insurance carrier.

If she refuses to compensate you voluntarily for any damage to your car, then you could  sue her: in a lawsuit, if you can prove (e.g. by witness testimony, including your own; by any physical evidence or photos) that 1) the other driver was at fault--that is, she was driving negligently, or unreasonably carelessly, in hitting you; and 2) that she caused the damage to your car; then you should be able to recover the cost to repair (you'll need repair estimates to prove the cost).

You have to prove these elements by a "preponderance of the evidence," or that it is more likely than not that things occured as you say. She can try to refute or deny you case with her own testimony or evidence; however, since there is a strong presumption that thhe rear car in a rear-ending accident was at fault, you should have a good chance of winning. The only way to get money from her if she won't pay voluntarily is to sue. One option is to sue in small claims court, where you can act as your own attorney and save on legal fees.