had u hit the old lady's car, the cop would have made the police report in no time. Don't get fooled by not calling ur insurance. The first thing is call the insurance and do wateva they say.  I am pretty sure u may have taken the officer's name in case u need later. You won't get any points for getting hit from back.  If she has the gut to run away after hitting u and even after admiting that, the cop is telling u not to file police report, something is going in back. So the right call is just call ur insurace right away and tell then everything including wat the cop told u to make the deal with the woman. 

You don't have to loose anything by calling insurance.