Gmail Credentials

Is it sin to use the Gmail contacts of a person if she had given me the credentials. Actually, I was in a relationship with a person and she dumped me and I got to know she is in a relationship with someone else now. So I did msg a few of her contacts saying that she is a cheater and she will dump them too. now she blackmailing me that she will go to cybercrime. She confessed in Facebook about her other affair and since I questioned her she did not accept this and she is backfiring me. Also, she should give me 3 lakhs which she took as help from me. Please kindly suggest.