<human>: Condense the text into a few key points

My girlfriend and I have been together for a bit over a year now, and we've had a great and fun relationship all the way. 

A backstory here is that we've regularly talked about our past and had a few discussions about it. We had a little argument about one guy she slept with (whole other story), but I got told she slept with him with a condom one year before we met. No biggie, I might have overreacted a bit when we argued and we've talked about that as well, so I guessed all is fine. 

A couple of days ago I asked for fun if she's spoken to any of the guys she's slept with after we started dating, and she said she didn't remember so she checked her Facebook while I looked.

She had some messages from two days before we "officially" got together, from that guy, asking if she has had her period, and that she wasn't pregnant. I pretty much panicked because I thought she had sex with him while we were dating, but she said she had sex with him in October with a condom. (This was during the first argument, and really, I'm not stupid but I didn't have energy to argue more). 

Yesterday I asked if she's lied about anything else, and after some back and forth she admitted to having sex with him a week before we started dating, without a condom. (This got me fired up because she lied while I told her what I feel about lying). 

That would mean about a month from they had sex till she got the message. She said she lied because she was afraid of my reaction after our first discussion about him. 

Is there anything I should do here? Or should I handle this like a "better man", and believe her and just move on? I have to note that we have had no issues like this earlier, and I have had no indication that I can't trust her before this.
<bot>: Girlfriend lied to me about some past sexual relations, and lied to me again when confronted and told that I really don't like lying. Am I overreacting or is there some kind of underlying issue here?