I agree with Jay, and will also note:

Stop giving a fuck about what your ex thinks or knows. You're entitled to move on with whoever you want.

Also, stop giving a fuck what this girl thinks as well. Start caring about yourself.

Just let it die off and back away from her, and if she gets mad and tells your ex or some shit, then lets be honest, you're better off without her anyways (and also, what's your ex guna do? It's not like you've fucked the girl (or have you?) ).