Unfortunantly, I do not own Harry Potter.
Right after/during when Lupin leaves Grimmauld Place on page 214 of deathly hallows.
"I'd never had believed this." Harry said, fuming. "The man who taught me how to fight dementors- a coward." He hissed the last word, and it felt like Remus had been punched in the stomach. Before he knew what he was doing, his wand was in his hand, and his spell hit Harry, throwing him back against the wall.
Lupin spun, and ran to the door. He hesitated a moment when Hermione yelled at him, but then he ran out, slamming the door behind him. He stood in the cold square for a moment, staring at the house. They didn't know. He thought, breathing heavily. They don't know. They're just kids. They don't understand. He turned quickly on his heel, and then disappeared.
He didn't think about where he was going. He appeared in front of Tonks's parent's house. It was dark, and none of the light were on. Lupin stood in front of the house for a long time, just staring at it, considering what would happen if he left, what would happen if he stayed.
Finally, he went to the door, his shoes squelching in the swampy grass. He took a deep breath, and stood in front of the door, his knuckles resting on the damp wood. If someone answered, he would stay. If someone didn't, he would never come back. He would let Tonks live happily with the unborn child.
He rapped three times, the dull thud barely penetrating the night. There was a small noise inside of the house, and then a light flicked on. Lupin held his breath, ready to run at any second.
The door swung open, and the moon was the only thing that illuminated Tonks's face. She squinted at Lupin for a second, and then her eyes widened. "Remus?" She whispered, staring at him. He was silent, stiff. His arm still slightly extended from knocking. Her mousy brown hair started to lighten on it's own accord.
She threw herself at him, smiling into his shoulder. "You came back." She said, and he wrapped his arms around her back. Her hair turned bright blue-pink. He pulled away from her, and she rubbed her eyes with her hand.
"Are you crying?" He asked, surprised.
She shook har head. "No." She said, with a watery smile. "Yes. Maybe." She laughed, and Lupin smiled sadly. "I just. I didn't know if you were ever going to come back." She said softly.
Lupin kissed the top of her head, his hands shaking slightly. "Of course I came back." He said, his hands moving down her back until they reached her waist. His hands touched the side of her stomach, and he felt a small bump that had not been there before. He drew back. "Oh my god." He whispered, and Tonks looked up at him, worried. He stared down at her.
"We're having a baby." He said, a smile coming to his lips.
Tonks nodded, her eyes rolling, and smiled back up at him. He felt dazed. She felt cold. Tonks walked back inside, gripping his forearm. She led him to her bedroom, and he sat nervously on the edge of her bed. "I don't think your parents would like it if we…" He said, trailing off. Tonks rolled her eyes again.
"Well, I am carrying your child." She said, and his eyes widened. He was afraid. There. He admitted it. She pulled him down onto the bed, and he lay stiffly against her. He was much warmer than she was, and she nuzzled into him. It was strange, her having a pregnant stomach. He was much older than him, and he had never seen this happening.
He was glad it had.