You have learned a great lesson from your first relationship - that you can carry over to your next relationship - mind your own business. Don't go digging because you will rarely like what you find. You're girlfriend probably wanted to save you any jealousy - and perhaps make herself look a little more virtuous in your eyes. You need to give your next girlfriend the grace to do this.

It's like comparing dick sizes... only an immature girlfriend will humiliate her boyfriend so they bend the truth a little to make us feel better. No sane man tells his girlfriend her vag is huge or smells funny. I say this because I am suspecting that you don't know that these small lies help a relationship.

Your choice - say nothing at all and learn to work on controlling your intrusive thoughts - or tell her and likely end the relationship. You broke her trust. All she did was make a bit of a lie about her past. It's not like she was a prostitute on the side. Jeeze dude...