Questions: Dating

So my girlfriend of almost four years (live together) has always talked about her teenage crush (how she liked his personality, loved hanging out with him, his physical traits and etcetera), they never did anything physical that I know of (kind of untouched chemistry the way she talks about him), they've been friends on FB, Snapchat and Instagram since I met her but they weren't talking until early December of last year. here's the start of my issue, there was secrecy behind it. How I know was during December my phone broke and I had borrowed her spar phone and went on a family trip for a week to go Christmas shopping (while she was at uni studying for exams) and I didn't have the money to fix my phone and shop for presents. I got multiple all cap texts popped up from her best friend basically saying "OMG _____ MESSAGED YOU?". I talked to my sister about it and he said I should respect her privacy but talk to her about it and I did. When I asked her about it and why she was so excited about it and all, I asked what he said and she said he basically just said hi (which I found suspect) so I asked if I could see the message and she said she deleted it because she was scared. Last week she said told me after she went out with her friend in the story at the bar that he messaged her and asked to if she wanted to hangout, she said "obviously I'm not going to but what do you think" I was a little awkward but I told her like it to her but please handle it respectfully. She was acting weird all night after that and I couldn't get it out of my head why for days until I went through her phone and found this

I don't like or find her response respectful. It seems like she gave him a the right away on a night I'm busy. She knows I would never be okay with her going out with another guy and I feel like she completely entertained it. I know I'm wrong, should I feel this way and what should I do? I kinda feel cheated on already. What should I do?