Did I have a right to get upset with my friend for taking forever to respond on yahoo messenger?

Everytime my friend and I are chatting on yahoo messenger, it starts off nicely by her giving me quick replies AT FIRST. When we've been on after ten minutes or so, I'll type something in and it will take her like 10 minutes to give me a reply. I will ask her if she is busy and she'll act like she wants to keep talking and then it might take 10 to 15 mins for her to respond. Usually, I try not to be rude and just keep saying 'u there?', she replies 'yea' five mins later, and it will just go on like this for hours. She won't even tell me when she leaves sometimes. I've put up with this for a long time, but last night was it. I asked her what she was doing. She's like, 'I'm talking to my boyfriend on the phone', for this ENTIRE 15 mins I'm sitting here waiting for a reply. I finally I harshly snapped 'You're being rude, I'm sick of it, I'm always dealing with it, etc.' She was really apologetic and super nice, but I feel so guilty. We've never fought before. Was I out of line or was she