The Twilight actress is said to have contacted the director hours after she was photographed being intimate with him.
"She was caught up in a forbidden relationship with a man who should have known better," a friend of Stewart's told The Mirror.
"Somehow things got totally out of control. Kristen had wanted to end their closeness that day, but her desires and emotions got the better of her.
"She was caught up in the moment and totally besotted by him. After that day she texted Rupert that she never wanted to do that again because she feared being caught and that things were too intense.
"She had no intention of leaving Robert and Rupert has never mentioned walking away from his family. She knew she did wrong, but realised how those photos would rip out Rob's heart. She has been in floods of tears since."
Copyright: WENNThe friend added that the news of the fling has "shocked" everyone close to Stewart and Robert Pattinson.
"Everyone feels for Rob on this. Kristen has shocked us all. If it would never have come out then everyone may have lived happily ever after," she said.
"Rob is seething and hoping the scandal can die down soon so he will return to his normal life. Rupert is doing his best to convince his wife this was a blip and they have a future."
Robert Pattinson is rumoured to want to meet Sanders's wife Liberty Ross in order to find "answers".