My girl and i broke up recently and she heard from one of my "friends" that i cheated on her. My "friend" just got dumped by her bf for cheating

She thinks i hooked up with this one chick i know and with "random girls at parties"- which ive never done in my life. I offered to call the girl she thinks and prove to her but she has a deep down feeling i cheated. And no evidence, being: hearing from the girl, seeing it, finding underwear or something like that.
How do i convince her that im innocent, ive been as rico suave as possiable. We have gone out for 1 year and 2 months. We have a VERY passionate deep relationship and are both truly in love, and the "spark" is still there.