Karin began to stroke her large cock, and began to feel the pleasure that came with it. Slowly, she began stroking it harder, and harder, even beginning to use both hands in the act. Moaning, Karin said, "Ooh... that feels so good..."
Karin didn't notice a small gurgling in her stomach, though. As she continued to masturbate, the gurgling grew stronger, and eventually a pressure began to form and grow. She was having too much pleasure to notice, though, although she did notice that it took longer than usual to reach orgasm. She continued to stroke her long member, all the while not noticing her expanding abdomen. It wasn't expanding with air, though. It was expanding with fat, coming from seemingly out of nowhere. Before she could increase an extra ten pounds, though, she reached orgasm, and cum shot out of her penis, splattering the walls of her nest. "Ooh... *giggle* you needed that didn't you?...", Karin said as she playfully talked to her dick.
Her member had finally gone flaccid, and shrunk back down... to nine inches? She looked at it again with pride, when she noticed this. It was longer than before, but she could tell it was completely flaccid. Standing up to examine it, she noticed something else too. She had gained a little weight, starting from her originally slim figure of 107, and having gone up to 118. It wasn't much, but it was interesting. "... This is interesting..." she said to herself.