Old girl friend asking ransom money

I am 40 year of age muslim man married to hindu girl. i had a extra affair with another muslim girl for about one year from 2013 to 2014. i had sex with her 4-5 times during this period, she was aware of my marriage status and never oppose on having sex, i used to give her gifts in return of sex (but not money) One day i was caught by my wife and my wife talk to her and me about it, i understand my fault and say sorry to her and things settle slowly. i stop talking to my GF after that time. Now after almost one year my old GF is threatening me to file a case against me in police and asking me for ransom, today she called my wife and introduced to some persons (father, mother and her boyfriend) and ask her to pay a big amount to close this. since last year i am not in contact with her but suddenly she did this. we used to chart in whatsapp and exchange pictures of each other often, she is telling to show those pictures as evidence against me, FYI no nude photos are exchanged. Please advise what should i do?