It's sizzling romance because she burnt his jumper, geddit? … Sorry.
In this chapter, Tonks thinks about Remus' hair and stands in a queue.
Two days later, Tonks found herself in a Muggle shop, faced with racks of clothes and no idea what to get. Since Diagon Alley wasn't known for their clothing - unless it was robes and pointed hats - she had apparated to east London after work in the hope of finding an appropriate replacement for the jumper she had destroyed, but was finding it more difficult than she expected.
Amusement had turned a little to embarrassment over the last couple of days. Being new to the order, and with Moody's recommendation, she was eager to prove herself just as reliable and skilled as everyone else. She was sure that she had proved her worth in fights, and obviously her ability to change appearance was an incredible benefit to the Order, but she continued to make these mistakes at Grimmauld place, tripping over and ending up flat on her face or...exploding people's clothes off. She especially kicked herself for letting it happen to Remus Lupin. While he was the one who would have been nicest about the incident (and he was characteristically lovely about it), he obviously was well respected in the Order. Having been a member during the first Wizarding War, the danger he put himself in for the cause, and magical ability he possessed, it was no wonder. Yes, it was definitely just respect for Remus that made her embarrassed at making so large a blunder in his presence and which was making her fret so much over what colour jumper to buy.
Tonks checked her watch. Seven minutes until she was supposed to be at Grimmauld place to go stake out a potential death eater hideaway with the man himself. Her eye caught a forest green sleeve poking out between the blues and browns. She plucked it off the railing and held it at arm's length. It was simple. Round neck, thick cotton, and a deep, earthy green. Tonks pictured Remus in it in her head, and she satisfied herself that it would look nice with his sandy coloured hair. Next, she held it up against herself. Remus was much taller than her, and as the jumper reached her mid-thigh, she thought that it must be long enough - and if it was slightly too short, well maybe that wouldn't be such a bad thing. The thought was in her head before she could stop it. Tonks had resolutely been trying to ignore the butterflies that flipped around in her stomach when she replayed the incident in her head. So what if she had found her gaze falling to his exposed skin a few times? It was only natural for the eye to be drawn there after someone's jumper had been burned off. So what if the image kept popping into her head of the line of soft brown hair trailing his down his stomach, cut across by the white line of a scar?
Five minutes. She rushed to the checkout, which had gathered a long line in the time she had been deliberating, then ran to the closest alleyway she could find to apparate to headquarters. Almost falling off the top step in her hurry, she wrenched open the door, panting slightly. Remus and Molly were talking in the hallway and started forward to meet her.
"We were beginning to worry, dear! Is everything alright?"
"No problems, Molly!" she said, then addressed Remus. "Sorry, got held up. Ready to go?"
She threw the plastic bag containing the jumper into the nearest room - there was no time for that now - and they left to apparate to the hideaway.
As it turned out, Tonks did not have chance to mention the jumper. When they got to their position, they were occupied in watching the suspected house as Death Eaters came and went. Their suspicions were more than confirmed. Identifying them was difficult in the gloom of the night and this took most of their concentration as they sat concealed by the grass and scrub and a disillusionment charm. Eventually when it seemed there was no more to see, at around two o'clock in the morning, they left the spot - they had been there to observe, not to fight them. Before they parted, Remus insisted on being the one to write to Dumbledore and report to the order on what they had discovered and implored Tonks to go home and sleep. She did not protest, and she was so tired that she had quite forgotten the gift she had left at Grimmauld Place, and did not mention it before she turned to apparate.