Remembering movie title help?!?

The main character was a girl who can detect lies. She's bored of the same routine her and her boyfriend do every day and she ends up Falling for his Bestfriend. I think his name was jonah? She even thinks that he lied to her one time so she questioned every time he told her he loves her because she was paranoid of "how will she ever kno" if she couldn't detect a "lie" he told. She thought her gift had vanished, or stopped working but turns out he wasn't lying.