I know you say you can’t tell her why you know, but I honestly think you should. You don’t want this to bug you forever and propose to a girl that could be potentially hiding something. Just say you were hanging out with Justine because you need help with a gift, so you know she wasn’t with her. And see what she says. Trust and communication are important and it would suck to know you propose to a girl who could be doing you wrong, but also just as bad to sit there and let that lie eat you alive.