I think you need to tell her and tell her that this is your child, her grandchild and she needs to take a step back. Do you live at home with your mum? I think you need to set the boundries before the baby comes along.
I am giving you this advice as when I had my first son I lived at home with my mum and dad and was also 19 when I had him. I was very lucky that my mum and dad gave me £1000 to help with baby stuff but they left me to pick everything myself. When I had Evan they made it quite clear to me that he was my child and they would help with the odd things but they never got up for night feeds or did the washing, ironing, etc for me or Evan which is the way I liked it we made these boundries before he arrived and after 3 months me and Evan moved into our own house (I was waiting to find a house beforehand and also was a little bit scared to move into a house with a newborn as I was a single parent).
Hope this helps a bit and now I am 25 and in a stable relationship with my second on the way.
If you want to ask me anything else just PM me or write on here, hope it all works out with you and your mum but just be strong and tell her otherwise you will be suffering in silence, remember its your baby and you make the decisions in its life not it's grandmother. xxx