Is it normal to be pissed off when your online chat partner leaves you hanging?

I have been chatting with this girl from another country for a few months and we have gotten very close and comfortable with each other as friends. She trusts and tells me a lot about her personal issues and I give her a lot of help and support. We have only met once in real life when I flew out to visit her.

Recently she moved to another country to pursue her studies. We're now in a different timezone and have continued chatting, me staying up late (her late afternoon) or waking up real early (her late night) to accommodate her.

So one Saturday night (my midnight & her Saturday late afternoon) I tried to start a chat with her. She returned my "hey" ... which meant she was online. I then told her what I did that night and asked "u busy? wanna chat?"

One hour went by and she did not answer or say anything. I got annoyed and sent some annoyed emoticons. 10 mins later she gave me a short reply "u drunk" ... and I thought wtf?

I said yes I drank a bit and asked if she was busy again. 15 mins later she says she's a little drunk too. We always had a "drunk call" thing going where we'd call each other while tipsy. So I suggested a drunk call. No response.

By this time it was something like past 2am my time, but about 7pm hers. I was exhausted and I gave up, really pissed off.

I had always told her that if either of us were busy, we'd tell each other outright so that neither would be left hanging. So I was pissed off because I expected her to have the courtesy to say "busy ttyl" and I'd simply go to bed.

I thought to myself it'd take less than a few secs to type "busy ttyl" Why couldn't/didn't she do that? Why leave a friend hanging? Is it unreasonable to expect a simple courtesy?