I imagine that your problem stems from the fact that you JUST found this out, and clearly this young man still has "access" to your girlfriend. That tends to make people feel more suspicious....."why did they wait to say anything?" "are there still feeling", etc...
Let me tell you something I learned from experience.
One - sometimes past relationships are not mentioned because there is a "fear" that someone will be hurt, and they don't want that to happen. They know they are "over" the past relationship, but worry that the current significant other will overreact.
Two - the past relationship was NOT a "soul" match and it literally just didn't occur to her to say anything. It was over and she simply didn't give it another thought.
It could be a combination of the two. She could be totally over it, but worry that you would worry, since the guy still flirts with her.
You cannot change the way he behaves, or the way she responds. The only person you can control is yourself. I would explain to her that you think he is flirting with her and ask that she not put herself in that position. That's all you can do.
Can I ask how old she is? The only reason I ask is that sometimes, before a girl really matures (emotionally), they really enjoy that attention.....even if they have no intention of following through with anything. They'll say "it doesn't mean anything to me", yet still appreciate the attention. If that is the case, it's not time to throw in the towel........but it is time to let her grow up a little more.
You're going to just have to pray through this. There is no magic solution for curing jealousy. Ask for God to help you express yourself to your girlfriend, then accept that you've done what you can do. It's not unusual to feel "jealous" when you find out that someone you care for once had feelings (even if they weren't strong feelings) for someone else.....but please keep in mind that NONE of her past "feelings" effects how she feels for you at all.
Just don't let your feelings of hurt and jealousy make you sin...that's the kicker.