The pair called an end to their five-year relationship earlier this year and at the time said the decision was mutual.
But reports now claim that Shayk dumped Ronaldo after finding messages on his phone from "dozens of women", reports mirror.co.uk.
"For so long she trusted him and stuck up for him but now she feels completely let down and betrayed," The Sun newspaper quoted a source as saying.
Further it was claimed that the 29-year-old, who is now reportedly dating Hollywood actor Bradley Cooper, received messages from anonymous women saying they had flirted with Ronaldo.
Their relationship reportedly came to an end after she checked his mobile phone statements and discovered numbers attached to women from across the world.
The source added: "She immediately confronted Cristiano about it, but he initially denied any knowledge.
"After she explained to him what she had found he eventually admitted to messaging the girls.
"She couldn't believe it so immediately got out of the house. She ended the relationship there and then."