Mother being over bearing.

I love my mom I do and I'm grateful for all she's done for me. Like letting me living with her and feeding me some times. Its just that she is so over bearing. I been planing to move out and soon I'll be able to but she keeps asking when and I keep telling her soon. Ever time I talk to her there are two things that come out of her mouth 1) when am I moving out and 2) how's my son. She worry's more about my son then me and it makes me fell like I'm not welcome and I'm taking up space. Then there's the problem of her doing stuff "for my sons benefit" which is her getting stuff or doing stuff with him without asking. Like buying a car seat that dose not work in the car I have so She'll have to use it in hers. I know she did it on purpose. Or buying his first Halloween costume without asking my sons dad or me. I get it, its her first grand baby. But she keeps crossing the line of grandma and mom. And I don't know how to tell her to back down. I nearly had a panic attack telling her to take back the costume she got my son. How do I tell her to back down and that I'm his mom not her?