So basically

My friend A is transferring from online school to normal school.
she's been at the school before but did online for a year.
she wanted to call for her last day without school.
So we called..
We were laughing the whole time.
It was a good conversation, I'd say.
Near the end she had to brb in the middle of something i was telling her about (which we were laughing at, so  I just wanted to get to the funny part so we can laugh more) and I waited about 10-15 minutes. 
She came back saying she had to sleep soon.
It was 9:57pm
I suggested we call for 3 more minutes.
So she said ok and I continued telling her about what had been interrupted previously.
Not even 4 words in and she says she's going to go to bed and that she wanted to end the conversation differently.
Then she hung up abruptly. 
I messaged her about it and told her we should call again and make up for it, yet again, I was still confused as to why she was upset. 
She went on about how I'm inconsiderate and should've known that she didn't want to keep the conversation going about the topic.