<human>: (together 7 months)
So here are the basics, my girlfriend and I are both young professionals and we are working in a city. Body wise, we are both slightly above average in fitness. No six pack, but I can run a 6:15 mile.

We have been dating for seven months now, and a bit about us sexually. My girlfriend told me that she doesn't sleep with just anyone, and that she only has sex in a relationship, and that she has only had sex with two guys before me. I am mentioning this because a lot of it is relative. 
 
So, I haven't introduced my girlfriend to my friends yet, I have posted a picture of us in the groupchat that we are a part of but that only has close friends. So now, I was looking through Instagram and saw that my friend Jake already followed my girlfriend. It threw me off since I didn't introduce him to her, so I asked him how he knew my girlfriend. 

He told me that he slept with her. And I'm like what, she never told me about you.  And he was like yeah bro, he basically just asked her to meet up off Instagram, and she did and she fucked him. And it blew my mind because he has screen shots of  the instagram messages between them. This isn't knew for him, he has done this type of stuff with other girls before, he is really good looking face wise, and he has a great body so I am not surprised. 

But I am surprised that he did this with my girlfriend.  Especially after that huge talk we had where she told me that she isn't like other girls who sleep around. Honestly, to me finding out about this really cheapens  the experience we had. Because,  it went from, oh I don't sleep with anyone, to I will jump into bed with a guy if he is good looking enough from Instagram. 

I don't want to break up, but this really does shake the foundation of our relationship. Am I overreacting about this? How would you guys feel about this?

Can you succinctly summarize the text in 1-3 sentences?
<bot>: found out my girlfriend slept with one of my friends off Instagram before we started dating, I am feeling uneasy about our relationship.