How can I retrieve Facebook messages?

[Iforgot]

I have a feeling my girlfriend is cheating on me and I got on her fb today & looked through her messages when we were hanging out. She snatched her phone away and deleted those conversations. She doesn't know that I know her password and have it on my phone but I can't prove it until I see those messages with those guys. If there's a way to retrieve them, please let me know. Preferably by phone but if that's not possible then a computer will do just fine. Thank you