Why does this happen?

We are in the middle of a convo. She's sending emojis, asking me questions, laughing at my jokes etc. and then she suddenly goes offline. It's happened a few times now and it's extremely frustrating.
I asked her out last week, but she said she was busy preparing for an exam and with her hobby. I respected the honesty and replied with "Alright, good luck with the exam and hit me up when you are done with it" to which she gave the thumbs up reaction and replied "thank youuuuu" finishing with that blushy smiling face.
Guess what, she didn't text me after the exam, so I asked her how it had gone and we went on to joke about and laugh, but suddenly she went offline. No words from her, hasn't even seen the last message. She's been putting the effort into the convo, it's not one of those "mhm" "yeah". I was about to ask her what she was up to for this weekend had she replied to my message.
I will ask her anyway, but if I get excuses without her trying to open up on when she'll be frew I will be too heartbroken to bother ever chatting her up again.
I sense that I could be irrational about the whole thing, but nevertheless I'd like to hear what you all think of the situation and maybe an input on how I could rationalise it.
Thank you very much.
Why does this happen?
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