It sounds to me that in public, where other people that you both know can see her, she's putting on an act. She can't shun you in public like that without looking bad in front of other people that she knows, and whose opinions matter to her.
On public transit, she can get away with shunning you, because there is no one there that she needs to impress. So she feels free to treat you like dirt. It also seems as if she was making fun of you.
NTs do this all the time, especially women. It's not all NT women, just the not-so-nice ones.
Don't feel bad, I've had these experiences in the past, especially as a kid and young adult. It took me a long time to learn how people are. It's not your fault that you are genuine and polite, and this person obviously isn't.