At What Point Is It Considered Being Used?

I love my mom and i want to help her out when i can. She asked me to help her move some furniture the other day but let me know that if i didn't want to that it was fine.

Today my grandfather ended up in the hospital so she asked me if i wanted to take care of my little sister for the night. Which i will so that she doesn't have to stay with a friend if she doesn't want to.

When having a conversation with my mother-in-law to be, she implied pretty heavily that i was being used. I don't feel like i am, i just feel like I'm helping my mom and these are legitimate reasons to do so right? And she made sure to mention each time that i could say no to her. How do you know when you're being used??