Help... Im getting bullied by an ex friend... what can I do?

ok so i used to be friends with this girl but i stopped bein friends wit her cos she was upsettin me about my ex boyfriend sayin he was cheatin on me wen he was wit me which i know for certain wasnt true .. and she was callin me a fat *** and just insultin me. so anyway i decided to go for a walk around the local park 2 evenings ago and i saw her(Nikki) and her own friend (Jen)who is really pshyco ,Jen started screaming at me and running after me saying she was gonna beat me up and i didnt say anything to her so i ran away from her back home...... then yesterday evening i brought my bro to the playground and nikki was there wit her cousin who i am aquaintances with. Nikki said to me ''Why were ya startin on Jen yesterday?'' and I said to her ''Well I who started that one'' (her), and she replied ''ARE YA STARTIN ON ME NOW'' and I just ignored her and she said ill knock ya out and then she said ''that shut her up''....... and she called me a ***** and a chicken for running away cos im not a fist fighter and I got beaten really badly unexpectadley a few months ago and was leftb lying on the road screaming and ever since then if anyone wants to start fightin with me i just run to protect myself. how can I stabnd up to her without punching her. I think she gets pissed off cos i run away so therefore she doesnt get a fight between me and her weirdo friend.