Coworker taking advantage of me

I have a coworker that ALWAYS asks me to take her shift or stay back, and doesn’t explain why. I always say yes because in my culture it’s considered rude to say no if someone asks you for a favor. I feel like she realized this and began to take advantage of it. I’m only part-time and I just don’t like how she never returns the favor for me. I’m so irritated 🙃