Ask why she's wearing a diaper.

"Why are you wearing a diaper?" you finally ask now that the two of you have some privacy.

"I..." she starts.

"Sorry." you say, realising that she's only be wearing them if she needed them. "That's a stupid question."

As you start the car, she remains silent.

"Do you always wear diapers?" you continue questioning.

She nods.

"And that's why you always stopped me when we were making out?" you ask.

She nods again.

She is silent for the whole drive and as you walk her up to her front door.