Maybe she just wants you to have better rest. You can be honest with her by saying you want to spend time with baby and will call for her when everything is done. Be grateful that there’s a family member who is willing to cook and look after your baby for you.
sometimes I feel the same towards my mil. when I'm w the baby, she keeps checking in on baby making me feel like I'm incapable of taking care of him. But I dun care alrdy. if she wants to care for him, ok go ahead. I just take it as a time for me to rest