Sounds like you two need to have a discussion about trust and boundaries. You don't trust her enough so you're looking at her phone -- why exactly? Has she given you reasons to not trust her? That is an invasion of privacy and is a no-no in a relationship. She should not be visiting an old boyfriend, however. And why is she messaging her old boyfriend? These things need to be talked about. Seems she was just angry about you looking at her phone, so she lashed out by saying she cheated. How is the marriage otherwise?