My first thought was I would be screaming at her to get her hands off my child! The issue is not the smoking here it is that you handled your child as any responsible mother would. I have grandkids that have gone to sleep in the cart lots of times! It is just life when you have kids. I take a small blanket with me and lay them in the cart when they fall asleep and put my groceries around them sometimes.
I never shopped much with mine when they were little they stayed in the car with their dad alot of the time. But when I was 23 I had 3 under the age of 3. 21 is not that young to have a child.
The lady was way out of line in harrassing you. She is entitled to her opinion but she is not entitled to take your child out of the cart!!!
You should have complained to the manager that she was threatening your child. I would have! A little one bobbing her head while asleep in a cart is no crime and it is not that bad for the child. I have seen mine bob their head the same way in a carseat! Yes it bothers me but sometimes you can't help it.
As for the car you were right there you were not miles away or leaving her in the car while you went and shopped or worse!
Hang in there! There are always people willing to tell you how to live your life!! Just ignore them! They need to clean up their own problems before they try to tell you what's wrong with you!
You are doing a good job with your daughter from the sounds of it so use your own judgement and you will be fine!