A simple "I'm flattered, but not really interested" would be sufficient.

She said she needed to go pick up her mom. Not sure whether or not she was being sincere. Now I'm afraid that she'll think I'm harassing her if I ask at another time or try to get a straight answer.

If she didn't give you s straight answer, I would wait a week or two and try again politely. You never know what's really going on. if you don't get s straight answer the second time time just move on.

Exact same thing happened to my son---all of his friends told him to forget about her but when he asked me I told him to pursue her if he really likes her. Long story short, they got married a few months ago and I really like her!

Sorry, couldn't help it.
People can be kinda crappy at times :(

Let's hope she has pepper spray and a whistle, so maybe that's direct enough for you. If a definitive yes is not there, walk away. Otherwise, you earn creeper status with her and her friends.