Ugh! I'm sorry that you had to be in the same place with that woman! Even if she never breastfed her children, heck, even if she never had children in the first place, I think she exhibited some rude behavior. I think this could go along with the discussion around WIM about women knocking each other down.
I was nervous to feed Squiggles in public the first few times I did it, which wasn't until he was over 2 months old. Before that, neither of us were good enough at it to try doing it in public! I had a similar experience in a McDonalds of all places. I was fully covered, too, but this woman just kept staring at me with this subtly horrified look on her face. I just stared right back with a big smile plastered on my face as if to say, "I'm feeding my child. What are you going to do about it?"
The only hiccup to our public feedings is that Squiggles is very much an 'open air' nurser: he doesn't really like to be covered with the blanket, and he swings his free arm around the whole time he's nursing (whether we're in public or not!). It's very cute at home, but when I'm trying to keep us covered, it's led to some near-nipple exposures!