2 issues - liability and damages.
On liability, if you rear end someone, odds are that you are the one in the wrong and liable for the accident, but you really haven't given enough info to judge.
On damages, if you get sued and the case goes all the way to trial, she has to prove beyond a reasonable doubt to a jury that she was injured. It's not hard to go to a doctor or chiropractor and claim that you have some sort of non-specific soft-tissue injury to your back and neck and be melodramatic about how you were hurt. Those kinds of injuries are also hard to prove and may be looked at skeptically by a jury.
Assuming you have insurance, turn it over to them, they will hire a lawyer to defend you and respond to her claim. This sounds like a routine case and they will know how to handle it, settle it, defend it if a case gets filed and so on. Try to develop a relationship with your agent and the lawyer to make sure they are paying full attention to your case.