Jessica Simpson claims Tony Romo broke up with her back in 2009 because he thought she was cheating on him with John Mayer.
Simpson spells out the whole thing in her new book "Open Book" -- in which she finally breaks her silence on the demise of her 2-year relationship with the former Dallas Cowboys QB.
"[Tony] accused me of seeing John behind his back. I hadn’t cheated on Tony at all, but I could not lie and say I hadn’t even seen him.," Simpson said.
Jessica told Tony "nothing happened" .