To confront her? Or not to confront her?

I recently found out that my girlfriend has been lying about one of her past relationships or hook ups (not sure how she classifies it). She had a pregnancy scare before I met her, and send an email to a guy telling him that she “knew it was his,” if she was pregnant and not some other guy whom she’d also had sex with in that month I guess. I asked her about it she swore she never slept with him, and never even saw him naked and he never saw her naked. A while ago I found out in a chat session with that guy the two of them were talking about an intimate session between them, not in great detail but things like “i was just focused on making you cum” and “i am a big boy if you remember ;-)” etc. I’ve asked her about the details of this relationship before and she swears that nothing ever happened. Should I confront her with this new information or not?