19:

Luna had an idea that something had happened between Luna and Draco when they finally met up again, four weeks after the ball. The professors had been bombarding them with work ever since they got back from the holiday, and it was the first time when all three had enough time to get together.
She noticed Ginny give Draco a small wink while they were undressing and the two seemed to share a secret. But Luna didn't think Ginny and Draco would do anything with out her, so she brushed the suspicion off.

Later on though, Luna couldn't help but think that something had gone on between them, and made her angry as well as jealous.
Well, if Ginny wanted a fight, she'd get it. She would just have to wait for the perfect opportunity- and in February, she got it.

It had been announced that Madam Pomfrey would not be at school on the first Saturday of that month as she was going to the Annual Healers and Matrons Conference to learn about a new potion to cure insomnia. Since it was flu season, every single person in the school received a Pepper- Up Potion before she left to prevent any sickness.

When Draco woke up that Saturday morning, Ginny's owl was waiting for him at the edge of his bed. How it got in his room he didn't know, but when he opened it up and read it the owl had already vanished again.
Meet me in the Hospital Wing. Wear your pyjamas and don't brush your hair. Sit in the bed on the far left corner of the room.
was all it said.
Curious, Draco threw his robes over his pyjamas and walked to the hospital wing when no one was around. He opened the door no one was there, but he did as the note told him and went to the furthest left bed and sat down. The curtain covered its side so that when Draco heard the click click click of high heeled shoes from far off, he couldn't see who was coming. Suddenly the door closed audibly and a key in the lock could be heard turning. The click click click of the high heels cam closer to where Draco was sitting and when Luna finally came into view, Draco couldn't believe his eyes.
Luna was wearing the highest white heels that Draco had ever seen and the shortest white skirt. Her shirt was small and also white, unbuttoned to revel her hot red push-up bra. On her head she wore a little white hat white a red cross on it; her blonde hair was pinned up in an elegant bun. Luna was dresses up as a sexy nurse.
"Hello Mr. Malfoy. Are we feeling any better today?" Luna asked in a sultry voice. Draco was just getting into the mood of things when Luna referred to him as "Malfoy" he was turned off.
"Is there anything I can do you make it better?" Luna asked. Pouting her lips and leaning over him to fluff his pillow. "Perhaps a massage?" her voice lowered as her hands trailed over Draco's button up silk pyjamas. His robe had been thrown off onto a chair nearby when Luna was locking the Hospital Wing door.
"Well, the nurse knows best. Administer any medicine you feel I need." Draco answered, helping her with the buttons.