Problem with my mom

About twenty minutes ago I mentioned I was going to take off work in order to be with a friend (weekend only job, I'm in high school) who was just diagnosed with depression. She immediately becomes what seemed to be angry and tells me I'm in the wrong and am self absorbed doing it for myself. This is a repeat occurrence she has some of the stupidest logic ever. How do you deal with a person like this it really makes me annoyed.