Should I feel bad?

So my coworker is pregnant, she's due next month. We work retail and she always calls out because she doesn't feel good. I'm always the person that has to cancel my plans and go in on an off day. Well today I told her no and I can't, and she got mad and called me selfish because I refused to go in for her. I'm really just tired of having to pick up her slack, and If she can't work then she needs to just quit or go on her maternity leave already, but she says she can't afford it. Idk now I feel like she's mad at me, but I just don't see it as my problem. Does that make me a bad person?