I wanna know what is going on in this situation.

Any opinion, comment, feedbacks are much appreciated. Thanks!

I am gonna get this as straight as I could, short and precise. The first time I started working in this job my boss was a very nice person, and so does until now. But for the past months things had been gradually declining in terms of my professional relationship to her due to the fact of misunderstanding. I expect so much from her as my boss, and by that I mean her taking responsibilities at any given situations and being smart. I have several comments on her and it is quite to mention. But personally I do think she's a nice person. Well recently the issue was me getting in trouble because someone put me in that situation where I was cornered. And that person who put me into trouble happens to be her casual buddy at work. To cut it short, I was so pissed and the fact that she didn't do anything about the issue or maybe I just don't know. Not that she mentioned anything to me either. Well come the other day I had a doctor's appointment where I had changed it several times until I had it finalized on the time that she wanted me to have (I had a convo with her about this before that incident/issue happened), and wanted me to come back to work later that day. She wasn't saying much but by the way she was talking to me about it she was implying in a pushy way for me to come back. Mind you we are all fully staff. The thing is I am all caught up with my work and have nothing much to do, besides my company pays us sick time each month and that time is everyone's right. Well the day on my doctor's appointment I was having a doubt of coming back because I was still upset since the past week when that issue happened, and I ended up not coming back to work that afternoon. I just called her back and told her I was struck in a traffic and still couldn't get out. Then the next day she was throwing tantrum to me all day and didn't wanna talk to me at all. She was mad as hell, I could tell, but wasn't saying anything probably due to the fact that she knew she can't do anything about it because I was using my sick time hours. She was throwing words at me in a subtle way also. My question is, why was she even mad about it in the first place? That's what's confusing me.