Your mom should have spoken with you before hand and you may want to bring it up to her that you didnt know she was coming and would have liked to talk about it before she comes so you can be prepared.
I know right now you may feel you dont need her or your old enough to do this and that she is doubting your ability. But its probably none of that at all. She may just want to be there for you emotionally and to share this experience with you.
Think of how you will feel when your baby has a baby one day. When I think of that it really makes me understand how my MIL and mother might feel. When I had DD my mother just watched me with her for hours. Every once in awhile she would tear up and I always thought she was amazed with the baby or that she didnt trust me to be alone with DD or that she was judging what I was doing or some crazy thing like that. Now I think it was she was amazed at the fact that her baby was a mommy. I know thats how Im going to feel.
And some times the emotional support from another women is something DH just cant provide. When I was having a hard time with breastfeeding my daughter I had a huge emotional break down. My MIL was kind enough (at 3am) to take the crying baby while I went and had a shower. And later when I was crying bc I had just given DD her first bottle and I felt so defeated my MIL just stroked my hair while I cried. She just understood in a way no male could.
And if she gets on your nerves have DH make a long and impossible grocery list and send her to the store since you wont be able to drive.