<human>: Hi. Long time lurker and first time poster. 

Me (21M) and my SO (19F) have had a rocky relationship for a long time now. Recently she spoke with her friends about this and got the recommendation about having an open relationship. 

At first I was against it, but after having a few talks about it we decided to try it out slowly. We both registered to a dating site and she immediately got a lot of replies from random people who wanted to meet & sleep with her.

At this point, I started feeling anxious about this whole thing. This is my first serious relationship (we've been together for about 1,5 years) so I spoke with her and we both uninstalled the dating app. 

Tonight, my SO went out with a few of her friends for a few drinks. After she came home, she almost immediately confessed that she had reinstalled the app to show her friend how it works (why would you need to install it on your own phone then?) and that another of her girl friends had been messaging different guys asking how they would fuck her, what they'd do to her etc. 

I asked her to show me what was written, but she refused saying that I was accusing her of cheating and that I don't trust her. I kept asking her to provide me with a reason why she wouldn't show me what was written, but all I got as reply was that she was ashamed of it (why be ashamed if you didn't do anything, right?). 

At this point we start arguing and it ends up with her crying, loads of bad words getting thrown around etc. She promised to delete the app and never install it again, but how can I trust that since she promised the same thing before?

I just need some advice on what to do. Am I overreacting? Is this OK behavior from her side? I mean, she knew that it'd hurt me if she went on that site again, but she did it anyway and justifies it with "I was just showing my friend how it works". 

Any help is appreciated. I feel like shit.

Could you distill the essence of the text into 1-3 sentences?
<bot>: girlfriend of 1,5 years went behind my back after agreeing to not have an open relationship and 'may' have sexted with guys on dating website. Not sure if I'm overreacting