What should I think about just finding out about a one night stand?

What if that was your past. I don't think that you would want a girl to judge you by your past. Past is the past we all have our share of experiences. That's just where she was in that time of her life.

Your first sentence said it. It was before you were together. As long as she is right for you now, what she did in the past doesn't matter.

That's why before you get involved with a girl, you need to come to terms with what is it that you consider acceptable behavior, traits, etc. People get older, wiser, etc. , and when younger, we also do 'foolish' things. If you think she has changed and maybe that was a one time thing, then no need to worry about it; gotta live in the present, not the past. But if you think this person is the same person from more than a year ago, well, possibilities exist.
Bottom line, do you see yourself marrying this person or having a long term relationship? If so, you better learn how to forgive and forget. If not, then better to start listing what type of girl you really want.

First off it happened "BEFORE" you started dating. Sorry but you had absolutely no ties or bonds between the two of you at that point in time. So what if she boinked him for 4 hours. I mean if she had fun great for her. Remember. You were not even in the picture yet.

This issue is not about her, but it's 100% ALL about you! You need to get over it, or get out of this relationship so she can find someone who isn't so hung up with her past. You're doing enough damage to this relationship all by yourself due to your jealousy and inability to get past something you had no control over.

Don't blame her for your insecurities.

Trust me, just let it go. What happened in the past can't be changed, so why drive yourself crazy over it? Has she given you any reason to believe that she would cheat on you?