Ask what she was so worried about.

Once you are in the privacy of your car, you ask "You were really freaked out back there. What were you so worried about?"

She takes a few moments to consider her response. Then, instead of answering, she asks a question. "I can trust you with a secret, can't I?"

"Of course you can." you reassure her.

"I wear diapers." She doesn't look at you as she says it.

Suddenly a lot of things make sense. "That's why we've never..." You trail off, unsure how to say it without being crass.

She nods. "There's more. Please don't freak out."

You take her hand to reassure her and wait for her to continue.

"I don't wear them because I need them. Although I've not used a toilet in so long I probably do need them now." She says. "Tanya treats me like I'm her baby. She keeps me in diapers."

"Why do you let her do that to you?" you blurt out.

Sarah blushes. "I like it. It's nice to be taken care of."

You drive home in stunned silence, unsure how to process this revelation. Once you arrive at Sarah's house, you walk her to the door.