Girlfriend hooked up with former friend. Should I feel some type of way ?

Hello everyone, although I've been reading on here for awhile I have never posted, this is my first one. It is a long one but i really appreciate all opinions and advice.....
So my gf and I have been together just about 3 months now. Its been amazing so far. This is only my 3rd serious relationship (I'm 21). She is really sweet and loving and we see each other just about every other day. I haven't been this crazy about a girl this fast ever. I guess u can call this crazy but we both feel we are in love with each other. But here's the problem...soo I used to be friends wot this guy who lives in my neighborhood (josh) he dated my sister for a couple years. We were like bro's. But wen they broke up 4 years ago we basically talked for 2 more weeks and then stopped (and it became a once in a while see u on the street say hi how are u doing conversation). My gf has a troublesome past as far as smoking weed alot and did a other drugs. She has been clean for 3 years. So last week we were walking and saw Josh and he sed hi and I stopped and sed hi and then he sed hi to my gf (i didnt know they knew each other) and he asked how she swas and she sed good and then asked how my family was and then we sed bye. After we walked away I sed "oh u knew him, I ised to be friends wit him he dated my sister a while back. And ALL she said was that "yea we smoked weed a few times and he used to see me n my friends on the avenue after i got out school". So I sed ok, but I had this hunch that they hooked up, a real gut feeling. Soo we saw him again 4 days ago wen me n my gf were walking to the store and he sed hi and we did as well. So 2 days later me n my gglf were talking and I just asked her, did u n josh hookup ? And she sed yea wen I was 16 (way before i knew her, now she is 21. I met her 5 n a half months ago). So I asked how many times and she sed they smoked and had sex once at his house. Do I have a right to be mad or upset that she didn't tell me about it til I asked her ?? Cause I know it was 5 years ago way before me, but still we see him from time to time walking the avenue like once a week and say hi n once had a brief convo beyond hi. I just feel like she hid it from tme being that we spoke about him a week ago and she didn't tell me they hooked up. I had to ask her days later for her to say they did. We have argued the last two days about it because she feels like she is not wrong for not telling me til I asked. And she feels she didn't hide anything. she feels she didn't have to tell me cause it has nothing to do wit her. But it has a bit to do wit me tho like I know that this guy was a sleaze back then after he broke up wit my sis. And the image of my current gf falling for him for a night and having sex wit him disgusts me and its like I just cant lookat her the same all of a sudden. The relationship had been going so well but these last two days have been awful, sshe's cried in front of me multiple times each of the past 2 days. I am not trying to be a jerk tho I hust feel she is wrong for not telling me considering we see him sometimes...I dont bring her in front of any1i hooked up wit in the past that she used to know (there is one girl we both went to schooland knew a bit that I hooked up wit ya few year ago)....soo what I am asking is do I have a right to feel upset and like she hid something ?? What should I do ? If I have to let it go how do I do so cause I haven't looked at her the same the last 2 days. I can't get the thought of them 2 hooking up out of my head. Any help is appreciated cause I feel I love her and it had been going so so well before this incident and idk what to do.