Saying no to friend at work

Long story short. I just earned the 7am to 3pm shift. I used to work 11am to 7pm. Now that I have the morning shift, it has allowed to to get a part time job-home health. I have 2 clients to visit after work. My friend at work asked me to switch shifts with her on a few days she selected. I explained to her how I have a job after work. She asked me to call in sick or say that I have car trouble. Is it selfish of me to say no? I actually cannot afford to miss any work.