At what point is a friendship taken advantage of?

I have a friend, actually, the friend I just mentioned in my last question, who just got out of a divorce and has two kids. I watch her kids every once in a while just to be nice. I do a lot of babysitting, but do not charge her, just because we’re friends. I just received a text from her asking if I can stay with her kids tomorrow night. It wouldn’t be a big deal except that it’s my b-day weekend and already took off work Monday, which she knows. Am I just being selfish or is she taking advantage of me. I’m new to having friends with kids, so has anybody else encountered this problem in a friendship?