Show her

"Alright madame."

You lift up a small portion of the dress by the side. Letting her see the small white knickers you're wearing. She felt frustrated as she couldn't see anything, she pulled your whole dress up and expose the front of your uncovered panties in front of her. She noticed your hard-on immediately..

"What is that hard thing you're hiding in your knickers. Pull down your knickers and show it to me."