How can people be so selfish

I am writing this in 'Red', because some people make me so mad. Without going into detail, I called my work to let them know I was coming in today, due to physical pain I have been having, but trying to ignore; but it got the best of me yesterday. I called 3 hours before my shift, and stated to shift manager that I was not coming in. In turn she tells the boss, when she got into work, and guess what; she calls and leaves a message, which said; "Merry the state is here today, and this leaves us in a bind, please call me back". So, I called her back and just told her, I didn't care if the President was there, I was in pain and could not come in, and I will bring her a doctor's note when I come back.....
Guess she wasn't too happy about that, but I have worked overtime, whenever someone is late or calls out, without saying a word.....Like I said, I've put my health 2nd the last time I worked here, and will "not" do it again..

Okay, that was good to get off my chest!!! Now I can breath..(until I talk to her tomorrow)..

Oh Well, such is life.