She's more secretive
According to Spalton, if your partner becomes increasingly secretive out of the blue it's one of the obvious signs she's got something to hide. He says: "She has her phone on silent vibrate and carries it with her all the time. She gets up when you're both watching television in the evening to go to the loo and takes her phone with her. She seems ages and gets agitated if you ask her if everything is all right."