Like others have said, it was another guy. It's always another guy. Right now all you can do is NC, but there is something you could have done after the "yea yeah, just busy" text she sent and this isn't a well known PUA technique or anything, but it's something I discovered and has worked for me.
There was a girl I met, hung out with a couple of times and then sexy time. After that I invited her to dinner with my friends and walked her home, everythings great and she seems into me. The next day I call her and she invites me this pub quiz thing, I accept and leave early to meet up with a friend who lives nearby the place. As I'm on a cab the following txt convo goes down:
Her: Hey, maybe you shouldn't come tonight, it's pretty boring. Not really your thing.
Me: It's okay, I'm already on my way to my buddys house who lives right next to it. Unless... you don't want me to come.
Her: Sorry babe, I'll see you next time.
Me: Haha, nah you won't. Look, I don't care if you **** other guys, just don't ever disrespect me. Good knowing you.
Afterwards she blows up my phone, says sorry and begs me to come. Saying she couldn't stop thinking about me. We turned into fbuddys and eventually had a pretty awesome relationship. At the time she claimed there wasn't another guy, but after we go together I found out she was seeing other guys and was going to meet up with another guy that night, but she cut them all out after what I said. You have to be able to back it up though when you say these kinds of things. I was fully ready to never see or talk to her again when I texted that.
So what did I convey there? I had always been super chill before this, never serious, just taking away attention when she misbehaves, but it this case, I had to show her I'm the kind of guy who knows how things work while at the same time set my boundaries. A guy who knows how things work is a guy who has been with a lot of women, and we all know they love that. How did I know it was another guy? It ALWAYS is in cases like these.
You got to the point where she's on the fence between you and another dude. You send her a needy text, BOOM, she swings to his side. You send another and she doesn't even bother answering. You could have done two things that would have shown higher value; NC, which works, but you might have to deal with the same bull**** later, or my method which gives you and instant result or closure, and you probably won't have to worry about her pulling that **** again. Mine didn't, in fact she wants to take a plane to come see me and travel with me this summer.
Example on how you could have applied to your sitch.
You: Are you alive?
Her: Yeah yeah, just busy with exams, don't worry.
You: Haha, riiight, "yeah yeah", really? Hey, it's cool if you want to **** other guys, but don't ever disrespect me. I'm not that kinda guy. I'm out.
See, she can deny that she was disrespecting you, but she won't, cause she knows it true and you calling her out like that shows her you're a real man and she'll start acting like a real woman. Now don't be getting all paranoid parrot on me and doing this everywhere whenever a girl is having a bad day. You need this only once, once for every girl. After that every time you go NC she'll know exactly why. Unfortunately, this is just for future reference. Right now, you're passed this and the best thing you can do is NC.