I '20M' think my (long distance) girlfriend '20F' might be seeing someone else. She messed up a few days ago but I'm not sure what to make of it nor what to do

Hello everyone. I'm making this post because...well I seek advice. (I'm in my early 20's, same as her) I am on an almost one year relationship with my long distance girlfriend. For context we meet at a comic con and after a while of talking on Instagram she asked me out, so I happily said yes. When she asked me out, we talked 24/7. Hours worth of calls, texting every day, playing Xbox together for HOURS and anything you would expect from a honeymoon phase. As months went by her replies got more and more stale. An example of this would be me asking about her day and her replying with "Good" Anything new? "No" One night I was playing with my friends and I saw she went live on Instagram. She sometimes went live and talked with friends from the convention I meet. Up to this point I drove hours to go see her and celebrate her birthday with her family and friends. I saw her again at another con and even ate after, which was not long ago, and we had an absolute blast! I joined but then the live ended. Mind you I texted her how she was and told her about my day with no reply. But the live was still up. I tried joining again, I saw a few seconds of the live, which was not a repeat like most livestreams that ended. Then the livestream disappeared on me. I thought this was a problem with my phone, so I asked my friend, who is friends with her as she talks to us, and he was able to see it. I joined and it was her with some random guy ive never meet. Odd, I thought. There was no more than 5 people on the stream. With my friend's account I told her it was me "your boyfriend" and she was joking with someone who commented "Stalker?" She said "emmmm stalker alerrt?" She was simply readying/ joking with the person who commented that who was a friend we also met at the con who was younger than us. on the livestream. I wasn't mad or anything just confused and maybe upset. After that she called me "You good? "What happened?" "Is everything OK?" I was a little upset so I said "yeah" and continued playing with my friends, after all what else could I do? She ended the live and tried to call me and texted "What did I do?" "It's making me extremely anxious knowing I made you mad" "Do you even want to talk to me?" I told her I couldn't as I was in my friend's house surrounded by his family while we were playing. Although that wasn't a lie I was also feeling cold, thirsty and had alot of anxiety. She texted "When are you headed home?" then claimed she was only going to be up for half an hour more as it was late. I told her we could always talk the next day if that was ok with her but she insisted in staying up and wait to "Know what's wrong" When we finished playing about an hour later I drove my other friend home, the one who's phone I used, and then drove home. She talks to him whenever we are on discord talking while playing video games. As I drove, she was texting him asking if I was angry and she was, from what he described, straight up panicking. From his description she was really miserable. I got home and I saw she was live with the same people. My friend saw the live for a few seconds and told me she looked miserable and she was crying. I wanted to join but he told me not to and it would be best if we both breathed and talked later. I texted her about it when she ended the stream a few minutes later. She claimed he was a friend, same as how I have friends. I texted her "I dont hide my friends from you" and next thing you know she wants to call. I informed her I couldn't as I would wake up the whole house, which was true. As we texted she showed me a screenshot showing that she wasn't hiding her live from me. The screenshot was a list of names and of course I wasn't selected, which lets be honest could have been deselected after the stream. I told her what I thought happened which, long story short, "You're talking to him hence your bland replies to me and when you saw me join you decided to hide me to avoid seeing this" She denied that claim and he is one of her best friends and "You know I have male friends" When I thought we were done texting by agreeing on talking the next day she texted me she couldn't sleep. I felt bad about leaving her with the thought that I was mad at her so I tried comforting her by making conversation about anything else to ease her mind. The conversation we had that night was like our honeymoon phase all over again. The feeling I got by having a normal conversation without a one word or a sad excuse of a sentence as a reply felt amazing. After she felt better by telling her I loved her and what not I went to bed. The next day we got on a call and she claimed it was a glitch because her Instagram was very buggy lately and she didn't hide the live. I told her in the most comforting way I could "If there's anything you want to talk about or anything you think I should know please tell me. If you think we are fading away or you feel like you lost interest I would like to know, I understand" She told me she loved me and it was nothing like that and she cried thinking I didn't trust her anymore. Skip to today and we have had conversations similar to when we just meet. Amazing wholesome conversations. She asked if we could move past this and I said "Yeah I trust you" But now I have no clue what to think. Sorry if this entire post reads like your 3rd grade essay you did at 10 in the morning while Mr. Jordan talked about his divorce, I just don't know what to do nor think. Also why doesn't she simply breakup with me IF she is cheating on me? I have no idea what I offer that makes her want to keep me. Am I a backup? Help pls & thank you all