Me(26M) get jealous/upset with current gf's(23F) summer fling with friend

Well a little bit of a back story. So 2 years ago me (24M) met this nice girl(21F) through a mutual friend and his gf, we'll call the Jon and Jane. We hit it off during that summer and basically had a relationship. We would go on dates, hang out, have sex. Pretty much all the relationship stuff without the status. She leaves at the end of the summer to go to a different school 5 hours away for college, which is whatever. We continue to talk and Skype all that stuff and then I offer to come visit her but she says no and that she doesn't want to continue this and just focus on school. Okay, I'm fine with that.

We still talk from time to time and when she comes home for holidays we usually will go out and grab a bite to eat and drinks, her idea every time.

Fast forward to next summer. Me(25M) and her (22F) and she wants to hang out during the weekend but I have a wedding so I invite her to be my date. She comes along with me, we go and have a good time but at some point during the wedding I look over and she is making out with a mutual friend, let's call him Frank, of me and Jon. I obviously get upset but say anything right away. I talk to her on the way home, I drove her there and told her how I was very upset that I would invite you as a date and then you go and make out with another guy. Told her if it was the other was she would be upset and she agreed that it wasn't right. Few weeks later it's Jon's bday party and she doesn't know how to get there so she calls me so she can follow me. I say fine whatever. Get there and then her and Frank disappear the rest of the night. Don't know what happened but I am sure we all know.

I leave it at that, obviously she has moved on.

During the winter I am hanging out with Jon, Jane, and Frank we got on the topic of d**k pictures and Jane said how Frank used to send them to that girl all the time so I assume that they fucked a bunch. During the holidays this girl would ask me to go for supper and drinks when she was back.

Now this summer, she (23F) comes back and wants to hang out with me (26M). I am fine with it and we end up hanging out a bunch, having sex, going on dates. I took her to a family wedding all that stuff. So the end of the summer comes and I just assume it's like all the other summers and she just wants a fling which I was fine with but she now tells me she wants a relationship. I think about it and agree to it because she is an awesome girl. Currently "officially" dating 2 months, been dating/sex for 4 months plus whatever you count as before.

I know get jealous/upset when I think about her and Frank the one summer. I have spoken to her about it and she said that she sees me as a dating quality guy and didn't want to hurt me or anything while she knew Frank was just for the summer fling but I still get jealous/upset. I currently liked her at the time and it hurt when she did what she did.

Am I being stupid by being jealous/upset ? Should I just let it go because she is with me now and obviously chose me to have a relationship with ? I currently also hang out with Frank from time to time which I think is why it bothers me so much. Me and Frank and also going to be groomsmen for Jon and Jane's wedding in the upcoming future.

TL;DR; Currently girlfriend had a fling with a friend one summer. I get jealous/upset when I think about it. Advice ?