Admit to what you were doing:

“I was trying to…” your voice gets quiet.

“Trying to what?”

“I was going to masturbate.” You want to die, this is by far the most embarrassing conversation of your life.

“You were masturbating while wearing my dirty bra and panties…” she pulled over along the side of the road, turning to look at you, with something between disgust and anger.

“I’m sorry mom…” you look down at your hands, resting over your smooth looking crotch.

She was silent for a moment, then pulled back onto the road and continued driving.