My guess, she wasn't sure, but she was worried you were preying on her ignorance and trying to fleece her. You see people who don't understand mechanical stuff fear mechanics, because they never really know if they can trust them. They don't feel qualified to call you out on a lie, but they're pretty sure you know that. She comes in to have her bars cut down, and you tell her she needs a full overrhaul. So how does she know she can trust what you say? If you do right by her, you may gain her trust, and earn a loyal customer, but the first time she walks through the door, she just doesn't know.

Oh, and women tend to feel this way more often than men, because they feel that merely by being a woman they are expected not to know about mechanical things, and therefore are targets for dishonest mechanics. Obviously women who DO know which end of a wrench touse don't have this fear, because if you explain it to them it makes sense, and anyway chances are if their brakes don't work they already know it.