Confused in what to do in this situation

I met this girl on penpalworld. we both live in Europe and we started texting. I don't usually do this kind of stuff, but i was bored and wanted to help people with English. Nonetheless, i met this girl and we started texting, somehow it went really well on the first day getting to know each other, constantly replying one message after the other, she replied straight after i texted and if not more. i was surprised how much she replied and talked. she even told me "not to delete her". we texted for like 4 hours or so to be honest and i have no clue how we did it. lol Anyways, this went on for three days in total. on Wednesday, we texted as well and she fell asleep during our conversation since it was late. (but she never replied in the morning). , but after that, on Thursday i texted her, and no reply whatsoever. (even though she has read the message as it tells me she has) Until Friday i texted her like normal again, she said she was busy studying and all and going out on friday night. Now i'm not so overly attached but i'm am damn more curious of why she or if any other girls do this? Or is this a game tactic thing? (even though it's only 3days ,i cant seem to understand it) what i want to know is, When she reads it, what goes in a girls head? do they just ignore people's text? is it that hard to reply back? or i'm just thinking too much. (not going to lie, but i was pretty intrigued for the two days where she didn't reply, like what is she doing to me lol) Sorry for the long details, i just wanted to know, what goes on a girl's head when they do this? I also think, if there was nothing going on, then why can't she just reply like normal or if i annoyed her or don't like me. why not tell me to delete her or she blocks me? Please give me some help and in details. Thank you for helping me out :)