Am i normal to be angry at my friend?

Hi, My friend recently quit her big job, to pursue another career. This career pays nothing. In order to pursue this, she needs someone to look after her children. I occasionally have babysat her kids, and she has paid me. fair. recently she asked me if i could look after her children twice a week for about 10 hours, I said i would have to look at my schedule to see if it was possible, but yes it might be. She then told me she could not afford to pay me. I then said i could not do it. Now she is all angry at me, and i am very very pissed off. I am disabled and suffer from chronic fatigue. any bits of work i do, i need to be paid. i think thats reasonable! i feel hurt , angry and sad. is this normal? i still want to be friends, i have tried to support her the best i can because i know her situation is not ideal, but i cant do this!